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SELECTION   IN  CLADOCERA  ON  THE 

BASIS  OF  A  PHYSIOLOGICAL 

CHARACTER 


BY  ARTHUR  M.  BANTA 


PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
WASHINGTON,  1921 


SELECTION   IN  CLADOCERA  ON  THE 

BASIS  OF  A  PHYSIOLOGICAL 

CHARACTER 


By  ARTHUR  M.  BANTA 


PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
WASHINGTON,  1921 


CARNEGIE  INSTITUTION  OF  WASHINGTON. 
PUBLICATION  No.  305. 


PAPER  No.  33  OF  THE  DEPARTMENT  OP  GENETICS. 


ANDREW  B.  GRAHAM  COMPANY 
WASHINGTON,  D.  C. 


CONTENTS. 


Page 

General  statement 5 

Source  of  material 6 

Culture  methods 8 

Pedigree  records 11 

Method  of  selection 13 

General  procedure 13 

Environmental  influences  and  reaction- 
time  17 

Test  series 18 

Same-day  broods 19 

Possible  criticisms  of  methods  of  selec- 
tion    20 

Nature  of  swimming  movements  of  Cla- 

docera  species  used  in  selection.  24 
Behavior  of  young  of  the  different  species 
when  released  in  the  experimen- 
tal tank 24 

Relation   between   environmental   condi- 
tions and  reproductive  age 26 

Statistical  treatment  of  data 26 

Analysis  of  data 28 

Line  695 28 

Presentation  of  data 28 

Analysis  of  data  for  effect  of  selection  38 
Relation  between  reproductive  vigor 

and  reactiveness  to  light .......  40 

Line  689 46 

Line  691 49 

Line  711 51 

Line  713 55 

Line  714 56 

Line  719 60 

Line  751 66 

Line  762 68 

Line  766 70 

Line  768 70 

General  introduction    for  Simocephalus 

exspinosus  lines 73 

Line  794 74 

Line  795 77 

Line  796 80 

Line  740 '. 84 

Line  757 89 

Presentation  of  data 89 

Detailed  analysis  of  data  for  Line  757  97 
Reaction-time  means   compared    by 

longer  periods 97 

Other  features  of  the  data 104 

Relation  between  relative  vigor  and 

mean  reaction-time ..                   .  108 


Page 

Analysis  of  data — Continued. 
Line  757 — Continued. 

Special  features  of  the  reaction-time 

curves 115 

Extent  of  the  change  in  reactiveness 

of  the  Line  757  plus  strain 116 

Reactiveness  of  both  strains  of  Line 

757  modified  through  selection.    118 
Conclusion  regarding  effect  of  selection 

in  Line  757 126 

Environmental    conditions    as    affecting 

reaction- time  and  vigor  of  stock  128 

1.  Temperature    influences 128 

2.  Influence  of  substances  exhaled  from 

observer's  breath 130 

3.  Relatively  temporary  chemical    (?) 

differences  in  water  used  in  ex- 
perimental tank 131 

4.  Occurrence  of    negatively    reacting 

individuals 132 

5.  General  influences  operative  through 

longer  periods  of  time 132 

Coincident  changes  in  reactiveness . .    133 
Seasonal  changes  in  reactiveness.  .  .    135 
Contemporaneous  shifts  in  reaction- 
time  means 135 

General  increase  in  reactiveness  of 

Simocephalus  exspinosus 137 

Independent  shifts  in  reaction-time 

means 140 

Whether  "  depression  periods  "  occur.  142 

6.  Environmental  conditions  as  affect- 

ing vigor  of  stock 143 

Reliability  of  the  reproductive  index.  143 
Coincident  fluctuations  in  vigor.  ...  J44 
Independent  fluctuations  in  vigor.  .  .    145 
Differences     between     reproductive 
indices  during  different  parts  of 

experiment 146 

Is  the  increased  reactiveness  for  lines 
of  Simocephalus  exspinosus  asso- 
ciated with  increased  vigor? ....    147 
Possible  divergence  in  vigor  between 
the  two  strains  of  a  selected  line  148 

Statement  of  results  of  selection 150 

Discussion  of  results 152 

Summary 163 

References .   169 


LIST  OF  FIGURES. 

Page 

1.  Line  695.  Reproductive  vigor 42 

A.  Average  number  in  first  brood 42 

B.  Average  age  of  mother  at  production  of  first  brood 42 

C.  Reproductive  indices,  actual  values 42 

2.  Line  695 43 

A.  Relative  rates  of  descent  of  the  two  strains 43 

B.  Reproductive  indices,  superiority 43 

C.  Reaction-time  curves 43 

3.  Line  689 48 

A.  Reproductive  indices,  actual  values 48 

B.  Reproductive  indices,  superiority 48 

C.  Reaction-time  curves 48 

4.  Line  691.  Reaction-time  curves 51 

5.  Line  711.  Reaction-time  curves 53 

6.  Line  713.  Reaction-time  curves 55 

7.  Line  714 58 

A.  Reproductive  indices,  actual  values 58 

B.  Reproductive  indices,  superiority 58 

C.  Reaction-time  curves 58 

8.  Line  719 64 

A.  Reproductive  indices,  actual  values 64 

B.  Reproductive  indices,  superiority 64 

C.  Reaction-time  curves 64 

9.  Line  751.  Reaction-time  curves 67 

10.  Reaction-time  curves  for  lines  of  Daphnia  longispina  and  composite  curves  for 

all  D.  pulex  lines 69 

A.  Line  762.  Reaction-time  curves 69 

B.  Line  766.  Reaction-time  curves 69 

C.  Line  768.  Reaction-time  curves 69 

D.  Composite  curves  for  all  D.  pulex  lines  with  curves  for  Line  751  super- 

imposed   69 

11.  Line  794 

A.  Reproductive  indices,  actual  values 

B.  Reproductive  indices,  superiority 77 

C.  Reaction-time  curves 77 

12.  Line  795 79 

A.  Reproductive  indices,  actual  values 79 

B.  Reproductive  indices,  superiority 79 

C.  Reaction-time  curves 79 

13.  Line  796 82 

A.  Reproductive  indices,  actual  values 82 

B.  Reproductive  indices,  superiority 

C.  Reaction-time  curves 82 

14.  Line  740 87 

A.  Reproductive  indices,  actual  values 

B.  Reproductive  indices,  superiority 87 

15.  Line  740.  Reaction-time  curves  with  reaction-time  curves  for  Line  757  super- 

imposed   

16.  Line  757.  Relative  rates  of  descent  of  the  two  strains 97 

17.  Line  757.  Reproductive  vigor 101 

A.  Average  number  in  first  brood 101 

B.  Average  age  of  mother  at  production  of  first  brood 101 

C.  Reproductive  indices,  actual  values 101 

18.  Line  757 102 

A.  Reproductive  indices,  superiority 102 

B.  Reaction-time  curves,  with  composite  curves  for  all  other  Simocephalus 

exspinosus  lines  superimposed 102 

19.  Line  757.  Reaction-time  curves  by  six-month  periods  with  similar  curves  for 

Line  740  superimposed 103 


LIST  OF  TABLES. 

Page 

1.  History  of  lines  of  Cladocera  used  in  selection  experiments 9 

2.  Data  from  an  early  test  series  of  Line  695  to  illustrate  differences  in  general 

reactiveness  to  light  on  successive  days 18 

3.  Summary  of  selection  data  by  broods  for  Line  695  plus 29 

4.  Summary  of  selection  data  by  broods  for  Line  695  minus 32 

5.  Selection  data  summarized  by  two-month  periods  for  Line  695  plus 35 

6.  Selection  data  summarized  by  two-month  periods  for  Line  695  minus 36 

7.  Selection  summary  for  Line  695 37 

8.  Same-day  broods.     Summary  of  data  for  Line  695 38 

9.  Selection  data  summarized  by  two-month  periods  for  Line  689  plus 46 

10.  Selection  data  summarized  by  two-month  periods  for  Line  689  minus 46 

11.  Selection  summary  for  Line  689 47 

12.  Same-day  broods.     Summary  of  data  for  Line  689 47 

13.  Selection  summary  for  Line  691 49 

14.  Same-day  broods.     Summary  of  data  for  Line  691 50 

15.  Selection  data  summarized  by  two-month  periods  for  Line  711  plus 51 

16.  Selection  data  summarized  by  two-month  periods  for  Line  711  minus 52 

17.  Selection  summary  for  Line  711 52 

18.  Same-day  broods.     Summary  of  data  for  Line  711 52 

19.  Selection  summary  for  Line  713 54 

20.  Same-day  broods.     Summary  of  data  for  Line  713 54 

21.  Selection  summary  for  Line  714 » 57 

22.  Same-day  broods.     Summary  of  data  for  Line  714 57 

23.  Selection  data  summarized  by  two-month  periods  for  Line  719  plus 61 

24.  Selection  data  summarized  by  two-month  periods  for  Line  719  minus 61 

25.  Selection  summary  for  Line  719 62 

26.  Same-day  broods.     Summary  of  data  for  Line  719 62 

27.  Selection  summary  for  Line  751 66 

28.  Same-day  broods.     Summary  of  data  for  Line  751 66 

29.  Selection  summary  for  Line  762 68 

30.  Selection  summary  for  Line  766 ".  70 

31.  Selection  summary  for  Line  768 71 

32.  Same-day  broods.     Summary  of  data  for  Line  768 71 

33.  Selection  summary  for  Line  794 74 

34.  Same-day  broods.     Summary  of  data  for  Line  794 74 

35.  Selection  summary  for  Line  795 78 

36.  Same-day  broods.     Summary  of  data  for  Line  795 78 

37.  Selection  summary  for  Line  796 81 

38.  Same-day  broods.     Summary  of  data  for  Line  796 81 

39.  Selection  summary  for  Line  740 84 

40.  Same-day  broods.     Summary  of  data  for  Line  740 84 

41.  Summary  of  selection  data  by  broods  for  Line  757  plus 91 

42.  Summary  of  selection  data  by  broods  for  Line  757  minus 94 

43.  Selection  data  summarized  by  two-month  periods  for  Line  757  plus 98 

44.  Selection  data  summarized  by  two-month  periods  for  Line  757  minus 99 

45.  Selection  summary  for  Line  757 100 

46.  Same-day  broods.     Summary  of  data  for  Line  757 100 

47.  Summary  of  data  for  more  reactive  individuals  of  Line  757 107 

48.  Comparison  of  reaction-time  means  for  the  plus  strain  of  Line  757  (within  which 

selection  was  effective)  with  corresponding  means  for  the  plus  strains 
for  S.  exspinosus  lines  in  which  selection  was  not  effective  (Line  740) 

and  the  shorter  S.  exspinosus  lines.     Lines  794,  795,  and  796 119 

49.  Comparison  of  reaction-time  means  for  the  minus  strain  of  Line  757  (within  which 

selection  was  effective)  with  corresponding  means  for  the  minus  strains 
for  S.  exspinosus  lines  in  which  selection  was  not  effective  (Line  740) 

and  the  shorter  S.  exspinosus  lines.     Lines  794,  795,  and  796 121 


SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF  A 
PHYSIOLOGICAL  CHARACTER. 


GENERAL  STATEMENT. 

For  more  than  8  years  (January  1920)  the  writer  has  been  rearing 
parthenogenetic  "pure  lines"  of  Cladocera.  The  original  object  of 
rearing  this  material  was  the  conducting  of  experiments  on  selection 
within  the  pure  line.  These  experiments  were  completed  in  May 
1917.  Data  bearing  on  other  problems  have  also  been  secured  from 
the  handling  of  this  material.  The  selection  experiments  only  will 
be  treated  in  the  present  paper. 

The  writer  undertook  the  selection  experiments  in  order  to  get 
additional  data  on  the  effects  of  selection  within  the  pure  line.  The 
Cladocera  material  was  chosen  for  several  reasons: 

1.  It  reproduces  rapidly. 

2.  Under  favorable   conditions  it   is  readily  handled   in   the 
laboratory. 

3.  It  reproduces  parthenogenetically  with  (under  carefully  con- 
trolled conditions)  no  possible  question  as  to  the  occurrence  of  sexual 
reproduction. 

4.  In  the  maturation  of  the  parthenogenetic  eggs  of  Cladocera 
there  is  a  single  division  without  reduction  (Weismann,  1886;  Kiihn, 
1908).     Hence  with  this  material  there  are  presumably  no  compli- 
cations of  segregation  during  maturation  or  of  fertilization  as  with 
sexually  reproducing  forms. 

5.  No  selection  experiments  had  been   made  with    a   purely 
physiological   character  as   the  basis    for    selection.      Aside    from 
the  fact  that  a  physiological  character  had  probably  not  been  used 
previously  as  a  basis  for  studies  on  the  effect  of  selection,  a  purely 
physiological  character  seemed  desirable  to  use  for  two  additional 
reasons:  (a)  Physiological  differences  are  frequently  readily  measur- 
able and  measurable  in  a  precise  way.     Numerical  series  of  measure- 
ments provide  very  usable  series  of  data  free  from  the  errors  of 
estimation  and  personal  equation  applying  to  series  not  directly 
numerical.     The  light  reactions  of  Cladocera  afford  a  character  per- 
haps as  definitely  measurable  as  any  readily  handled  physiological 
characteristic,     (b)  In  both  the  ontogenetic  and  phylogenetic  history 
of  organisms  physiological  modifications  may  occur  without  observed 
morphological  changes,  and  in  the  modification  of  organisms  physio- 
logical modifications  may  readily  precede  distinguishable  or  measur- 
able morphological  changes.     This  .latter  consideration  (b)  had  the 
greatest  weight  in  determining  the  use  of  a  physiological  character 
as  the  basis  for  these  experiments. 


6  SELECTION   IN    CLADOCERA    ON    THE    BASIS    OF 

The  rapidly  breeding,  easily  propagated,  parthenogenetic  organ- 
ism possessing  a  readily  measurable  physiological  character  afforded 
just  the  material  desired. 

The  experimental  object  was  to  attempt  to  isolate  by  selection, 
through  a  number  of  generations,  a  strain  more  responsive  to  light 
than  a  second  strain  from  the  same  pure  line1  selected  for  its  reduced 
responsiveness  to  light.2 

SOURCE  OF  MATERIAL. 

The  Cladocera  material  to  be  considered  in  this  paper  was 
obtained  from  ponds  near  Cold  Spring  Harbor.  Pond  I  is  a  surface- 
water  pond  in  an  open  lot  at  an  altitude  of  about  160  feet  above  tide- 
water. This  is  a  temporary  pond,  some  60  by  40  feet  in  dimensions 
when  moderately  filled,  and  contains  water  only  from  early  winter 
to  May  or  somewhat  later  in  the  summer.  Pond  II  was3  a  permanent 
spring-fed  pond  in  a  shaded  situation  in  the  woods  on  a  northeast 
slope.  It  was  about  one-fourth  mile  distant  from  Pond  I,  at  an 
altitude  of  about  60  feet,  was  approximately  18  by  10  feet,  and  had 
a  depth  of  only  10  inches.  Pond  III  is  another  surface-water  pond 
on  the  upland,  three-eighths  of  a  mile  from  Pond  I.  It  overflows 
occasionally  after  excessive  rains  and  frequently  becomes  dried  up  in 
summer.4  At  its  maximum  it  is  approximately  65  by  40  feet,  but 
it  ordinarily  contains  less  than  a  fourth  of  this  area  and  does  not 
exceed  2  feet  in  depth  at  its  deepest  portion.  This  pond  is  near 
and  receives  the  surface  drainage  from  a  barnyard  and  a  pasture  lot. 
Hence  it  is  rich  in  organic  solution  constituents.  Pond  IV  is  also  a 
shallow  surface-water  pond.  It  is  on  the  upland,  an  eighth  of  a  mile 
from  Pond  I,  ordinarily  has  an  area  of  perhaps  half  an  acre,  and  has 
become  dry  only  once  in  the  past  7  years.  This  pond  also  receives  the 
surface  drainage  from  a  barnyard,  but  because  of  its  larger  drainage 
basin  it  is  not  so  rich  in  organic  solution  constituents  as  Pond  III. 

The  Daphnia  pulex  material  used  in  the  selection  experiments 
was  obtained  from  Pond  I  (November  1911  and  October  1912)  and 
from  Pond  II  (November  1911).  Only  two  lines  (689,  751)  from 
Pond  I  were  retained  very  long.  The  remainder  of  the  lines  of 
D.  pulex  used  in  these  experiments  came  from  Pond  II,  the  small 
spring-fed  -pond  in  the  woods.  The  D.  longispina  used  in  selection 
were  derived  from  stock  from  Ponds  I  and  IV.  Collections  were 

1  These  were  not  "pure  lines"  in  the  narrowed  sense  of  Johannsen's  definition,  but  were 
clones  in  the  sense  adopted  by  Johannsen  and  his  followers.  The  term  "pure  line"  has  not  in 
the  general  mind  been  restricted  to  Johannsen's  usage,  however.  Nevertheless,  to  avoid  loose- 
ness of  expression  and  possible  misunderstanding,  it  might  be  wise  to  abandon  this  term  to 
Johannsen's  limited  meaning  and  for  a  general  term,  embracing  the  Johannsen  pure  line  and  the 
clone,  to  adopt  the  term  pure  lineage  or  pure  descent. 

*Two  short  notes  relating  to  this  series  of  experiments  have  been  previously  published 
(Banta,  1913,  1919).  The  references  there  made  to  Simocephalus  vetulus  should  read  Simo- 
cephalus  exspinosus  =  vetulus. 

*  It  has  since  been  drained. 

4  It  has  become  empty  in  the  autumn  three  times  in  the  last  7  years. 


A   PHYSIOLOGICAL   CHARACTER.  7 

made  in  October  1912,  in  Pond  I,  and  in  Pond  IV  in  November  1913. 
The  Simocephalus  exspinosus  material  used  in  selection  experiments 
was  obtained  from  Pond  IV,  August  1912  and  December  1914,  and 
from  Pond  I,  October  1912. 

The  stock  obtained  from  Pond  I,  the  surface-water  pond,  had 
in  all  probability  only  recently  undergone  sexual  reproduction.  The 
pond  ordinarily  remains  dry  from  early  summer  until  October  or 
November.  If  as  much  as  3  to  5  weeks  had  elapsed  between  the 
filling  of  the  pond  and  November  16,  when  the  latest  collection  from 
this  pond  was  made,  the  daphnids  could  have  descended  at  most 
only  one  or  two  generations,  allowing  several  days  for  the  (fertilized) 
ephippial  eggs  (produced  before  the  pond  became  dried  up 'in  the 
spring)  to  develop  and  2  weeks  for  each  generation  at  out-door 
temperature  at  that  season.  It  is  indeed  quite  probable  that  the 
individuals  collected  were  themselves  ex-ephippial  individuals. 
There  is  no  safe  criterion  for  determining  how  long  the  material 
obtained  from  Pond  II  (the  spring-fed  pond)  may  have  reproduced 
parthenogenetically  since  the  stock  had  last  undergone  sexual  repro- 
duction previous  to  its  being  taken  into  the  laboratory.  However 
in  this  pond  and  in  other  small  ponds  in  which  this  species  has 
been  observed  it  has  not  ordinarily  been  found  to  occur  for  more 
than  3  to  5  weeks  at  a  time,  so  that  probably  this  material  had 
comparatively  recently  descended  from  ephippial  eggs.  There  is 
likewise  no  way  of  determining  how  recently  the  stock  collected  from 
Pond  IV  had  undergone  sexual  reproduction.  D.  longispina  occurs 
in  this  pond  occasionally,  and,  so  far  as  observations  go,  seems  not 
to  continue  there  long  at  a  time.  S.  exspinosus,  however,  is  found 
there  the  year  round.  No  males  or  sexual  eggs  of  either  species  have 
been  found  in  this  pond.1 

The  five  lines  of  S.  exspinosus  originated  from  five  different 
mothers  collected  from  two  ponds  and  at  three  different  times. 
There  is  some  evidence  (see  page  123)  that  Line  757  (from  Pond  I, 
October  1912)  at  the  beginning  of  selection  differed  in  its  reactive- 
ness  from  Line  740  (from  Pond  IV,  August  1914).  The  progenitors 
of  these  two  lines  were  obtained  from  different  ponds  and  with  a 
time-interval  of  about  two  months.  The  other  lines  (794,  795,  and 
796,  from  Pond  IV,  December  1914)  of  this  species  seemed  not  to 
differ  in  their  reactiveness  from  each  other  or  from  Line  740  obtained 
two  years  earlier  from  the  same  pond.  Possibly  these  four  lines 
belonged  to  the  same  clone,  while  Line  757  belonged  to  a  different 
clone.  It  is  possible  that  sexual  reproduction  may  not  occur  in  the 
pond  from  which  these  four  lines  (740,  794,  795,  and  796)  were 
obtained,  and  they  may  all  have  come  from  a  common  progenitor. 
While  the  material  was  not  examined  with  this  point  in  mind, 

1  For  the  purpose  of  these  experiments  it  is  presumably  quite  immaterial  at  what  time  the 
last  previous  sexual  reproduction  had  occurred. 


8  SELECTION   IN    CLADOCERA   ON   THE    BASIS    OF 

between  60  and  100  collections  of  S.  exspinosus  material  from  this 
pond  were  made  at  intervals  of  several  days  and  no  males  or  ephippial 
females  were  found.  Males  might  readily  be  overlooked  in  col- 
lections not  carefully  examined  microscopically,  but  ephippial 
females  are  unlikely  to  be  overlooked  by  one  who  is  accustomed  to 
handling  Cladocera  material.  However,  sexual  reproduction  in 
Cladocera  is  spasmodic  and  frequently  of  short  duration,  so  that  the 
results  of  the  examination  of  the  collections  do  not  entirely  preclude 
the  possibility  (though  making  it  seem  improbable)  that  sexual 
reproduction  may  actually  have  occurred  during  the  period  of  fre- 
quent examinations  of  material  from  this  pond. 

The  same  general  reactiveness  appeared  to  exist  in  all  D.  pulex 
lines  (see  figs.  2c,  3c,  4,  5,  6,  7c,  8c,  9,  and  10D),  although  they  came 
from  two  ponds  (Pond  I  and  Pond  II),  were  collected  at  different 
times,  and  presumably  may  have  belonged  to  more  than  one  clone. 
Sexual  reproduction  in  this  species  was  several  times  noted  in  a  nearby 
pond  (Pond  III)  from  which  the  culture-water  was  obtained. 

Table  1  shows  in  condensed  form  certain  data  concerning  the 
lines  of  Cladocera  reared  in  the  laboratory  and  used  for  the  selection 
experiments. 

CULTURE  METHODS. 

The  culture-water  in  which  these  animals  were  reared  was  at 
first  obtained  from  the  spring-fed  pond  (Pond  II).  Beginning  a  year 
and  a  half  later,  it  was  gotten  from  Pond  III,  the  barnyard  pond. 
This  pond  receives  all  its  water  from  the  surface  drainage  of  a  small 
pasture  lot  and  from  a  barn  and  barnyard  in  which  live-stock  is  kept. 
The  amount  of  organic  matter  carried  into  the  pond  by  the  surface 
drainage  is  relatively  large  and  the  water  is  rich  in  organic  solution 
constituents.  The  water  is  usually  heavily  colored  a  reddish  or 
yellowish  brown. 

In  being  collected,  the  culture-water  is  dipped  up  in  a  manner 
to  obtain  as  much  as  possible  of  the  loose,  fluffy  sediment  from  the 
bottom  of  the  pond.  After  being  brought  into  the  laboratory  it  is 
strained  through  fine  "India  linen. "  The  lighter  portion  of  the 
sediment  is  gently  rubbed  through  the  straining-cloth  and  the  coarser 
residue  is  discarded.  The  strained  water  is  then  allowed  to  stand 
for  two  days,  when  it  is  again  (sediment  included)  strained  through 
India  linen.  After  a  second  interval  of  from  one  to  several  days  the 
water  is  thoroughly  stirred  and  a  third  straining  is  made  through  a 
standard  silk  bolting-cloth  having  130  meshes  to  the  inch.  The 
water  is  then  ready  for  use  as  culture-water.  The  first  straining 
removes  any  Cladocera  or  Copepoda  which  may  be  in  the  pond- 
water  and  removes  most  (at  least)  of  their  eggs.  The  second  strain- 
ing removes  any  young  which  as  eggs  may  have  passed  through  the 


A   PHYSIOLOGICAL   CHARACTER. 


9 


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10        SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

cloth  on  the  first  straining,  they,  meantime,  having  had  time  to 
develop  before  the  later  straining.  The  third  straining  is  probably 
quite  unnecessary,  but  is  used  as  an  added  precaution. 

The  culture-water  contains  a  small  amount  of  the  original  (now 
thrice-strained)  sediment,  has  much  material  in  suspension  and  in 
solution,  and  is  dull  brownish-gray  in  color.  It  settles  and  clears 
slowly,  so  that  young  daphnids  placed  in  the  culture-bottles  are 
visible  only  with  difficulty  for  a  day  or  two. 

The  animals  when  collected  were  isolated  in  200-c.  c.  wide- 
mouthed  bottles  filled  to  a  depth  of  near  5  cm.  (about  100  c.  c.  in 
quantity)  with  culture-water.  The  young  of  the  first  brood  from 
the  wild  mother  were  transferred  during  the  first  day  of  life  to  indi- 
vidual bottles.  A  single  individual  each  was  placed  in  the  No.  1  and 
the  No.  2  bottles,  while  three  or  more  were  placed  in  a  third  bottle 
designated  as  a  " prime''  bottle  to  serve  as  reserve  stock.  Transfers 
were  made  in  like  manner  in  subsequent  generations,  except  that 
Sunday's  broods  were  transferred  Monday,  when  one  day  old.  The 
bottles  of  the  parent  generation  were  retained  as  additional  reserve 
stock  until  a  second  descendent  generation  was  obtained.  The 
grandmother  generation  was  then  discarded,  a  few  individuals  being 
preserved  for  morphological  study  if  later  desired.1 

For  many  months  after  the  cultures  were  begun,  and  at  frequent 
intervals  afterward,  large  samples  of  the  culture-water,  after  its 
three  strainings,  were  set  aside  in  glass  jars.  Observations  were 
made  to  determine  if  daphnids  of  any  sort  developed  in  this  water 
from  eggs  passing  through  the  straining-cloth  or  in  any  way  introduced 
by  accident  during  its  handling.  None  appeared  at  any  time. 

In  addition  to  the  three  strainings  of  the  culture-water  and  the 
general  precautions  observed  in  handling  the  stock,  the  system  of 
handling  the  material  in  itself  served  to  check  out  any  possible  con- 
tamination. The  No.  1  and  No.  2  bottles  each  contained  a  single 
individual  and  the  prime  bottles  three  or  more  individuals.  Any 
contamination  must  have  been  detected,  except  in  the  case  of  an 
individual  of  the  same  species  and  very  nearly  the  same  age  in  a  No. 
1  or  No.  2  bottle  in  which  the  individual  belonging  in  the  bottle  died 
within  a  day  after  having  been  placed  therein.  The  likelihood  of 
the  coincidence  of  the  introduction  of  an  individual  of  the  same  age 
and  of  the  same  species  into  a  bottle  in  which  the  rightful  individual 
died  within  a  day  is  so  slight  as  to  arouse  little  concern.  If  contami- 
nation occurred  in  a  prime  bottle,  to  escape  ready  detection  it  would 
have  to  be  an  individual  of  the  same  species  and  of  very  nearly  the 
same  age  as  the  individuals  belonging  in  the  bottle.  Further,  the 

1  In  case  the  No.  1  individual  is  lost,  young  are  transferred  from  the  No.  2  bottle,  or  if  the 
No.  1  and  No.  2  are  both  lost,  the  transfers  are  made  from  the  prime  bottle.  In  case  the  prime 
too  is  lost,  extra  transfers  are  made  from  the  first  or  a  later  brood  of  the  No.  1  or  other  surviving 
individual  of  the  parent  generation,  and  young  from  one  of  these  transfers  are  used  to  start 
the  new  generation. 


A   PHYSIOLOGICAL   CHARACTER.  11 

chances  would  be  3  to  1  (since  there  are  three  or  more  individuals 
belonging  in  the  prime  bottle)  against  young  from  the  strange  indi- 
vidual being  used  in  propagating  the  line  in  the  unlikely  contingency 
of  this  bottle  being  used  to  continue  the  line.  These  facts,  combined 
with  the  fact  that  the  prime  bottles  were  used  only  occasionally 
(when  the  No.  1  and  No.  2  individuals  had  died)  in  propagating  the 
strains,  make  the  chances  greatly  against  the  perpetuation  of  a  con- 
tamination of  a  pure  lime,  if  such  a  contamination  ever  did  occur. 
With  a  single  exception,1  which  was  readily  detected,  we  have  every 
reason  to  believe  that  our  selection  cultures  did  not  at  any  time  be- 
come contaminated  by  the  introduction  of  daphnids  in  the  culture- 
water  or  by  any  other  means.2 

PEDIGREE  RECORDS. 

All  the  descendants  of  each  of  the  "wild"  individuals  originally 
taken  into  the  laboratory  are  referred  to  as  belonging  to  a  single  line, 
while  any  subdivisions  of  the  stock  within  this  line  are  designated  as 
strains.  Thus,  of  each  of  the  lines  in  which  selection  on  the  basis 
of  light  reactions  was  made,  there  were  plus  and  minus  strains,  while 
of  certain  lines  there  were  several  additional  strains,  all  of  the  strains 
of  any  line  of  course  having  descended  in  the  laboratory  from  a  single 
"wild"  individual. 

In  the  records  each  line  is  referred  to  by  a  certain  arbitrary 
number.  The  generation  of  its  descent  since  having  been  brought 
into  the  laboratory  is  indicated  by  a  letter  followed  by  a  subscript. 
The  plus  and  minus  strains  are  indicated  by  the  plus  or  minus  signs. 
Thus  695  DH—  indicates  reference  to  the  264th  generation  (the  letters 
of  the  alphabet  having  been  used  ten  times  over  and  to  the  letter 
D  on  the  eleventh  time)  of  the  minus  strain  of  Line  695. 3 

Notes  were  made  on  loose-leaf  sheets,  a  sheet  being  used  for  the 
notes  for  the  three  bottles  (No.  1,  No.  2,  and  "prime")  of  each  gener- 
ation of  each  strain.  Records  on  this  sheet  include  the  pedigree 
designation,  the  pedigree  of  the  mother  from  which  the  young  were 
derived,  and  the  date  of  birth  of  the  young,  all  made  at  the  time  of 
making  the  transfer.  Later,  the  date  of  occurrence  and  (usually) 

1  In  one  case,  early  in  the  course  of  these  experiments,  a  strange  daphnid  did  appear  in 
one  of  the  culture-bottles.    It  was  a  small,  rapidly  reproducing  form,  the  eggs  of  which  might 
conceivably  be  pressed  through  the  straining-cloth,  young  developed  to  maturity,  and  eggs  of 
the  next  generation  produced  before  the  second  straining.    It  was  readily  recognized  as  an  inter- 
loper.   Such  an  appearance,  even  had  it  been  that  of  a  stray  individual  of  the  species  under  cul- 
tivation, would  have  been  readily  detected  and  leads  to  no  great  concern  as  to  the  purity  of  our 
cultures. 

2  In  a  few  instances  two  individuals  were  found  in  a  No.  1   or  a  No.  2  bottle,  due  to  the 
lifting  out  in  the  pipette  of  two  (instead  of  one)    individuals  from  the  mother's  bottle  or  from 
the  expe  imental  tank  when  making  the  transfers.    The  prompt  finding   of  the  extra  individual 
added  to  our  confidence  in  the  purity  of  our  cultures,  but  in  every  case  the  bottle  in  which  an 
extra  individual  was  found  was  discarded.     Since  in  these  cases  the  extra  individual  was  not  of 
another  strain,  but  merely  an  extra  individual  of  the  same  strain,  its  presence  in  the  bottle  did 
not  represent  a  threatened  contamination. 

3  It  is  to  be  noted  that  the  minus  strains  are  strains  selected  for  reduced  reactiveness  to 
light.     No  negatively  reacting  strains  occur. 


12 


SELECTION   IN   CLADOCERA  ON   THE  BASIS   OF 


the  number  of  young  in  the  first  brood  and  the  date  of  occurrence 
of  the  second  brood  from  the  No.  1  mother  were  recorded.  The 
transfer  of  young  to  the  next  descendent  generation  was  noted  for 
the  appropriate  mother,  as  was  also  the  later  discarding  of  the  bottles 
and  the  preservation  of  individuals  from  the  No.  1  bottle.  The 
record  of  transfers  made  was  given  on  the  mother's  as  well  as  the 
daughter's  sheet.  This  double  checking  served  as  a  safeguard  in 
case  of  any  deficiency  or  discrepancy  in  the  notes.  Similar,  but  less 
complete,  records  were  made  for  the  No.  2  and  prime  bottles. 

The  data  obtained  in  conducting  the  light  tests  for  determining 
the  individuals  to  be  selected  was  recorded  in  a  separate  note-book. 
Any  discrepancy  as  to  the  number  in  a  given  brood,  its  pedigree 
designation,  date  of  birth,  etc.,  could  again  be  checked  by  reference 
to  this  book,  so  that  in  reality  a  three-entry  system  of  note-keeping 
was  utilized.  Very  few  discrepancies  or  deficiencies  in  the  notes 
have  been  found,  and  these  were  readily  cleared  up.1 

A  sample  sheet  of  the  loose-leaf  notes  is  appended,  and  also  a 
page  of  the  experimental-room  notes. 


Copy  of  pedigree  sheet  of  loose-leaf  notes. 

Copy  of  page  from  experimental  note-book. 

695  F6-  No.  1. 

695 

F6-  No.  1. 

10/20/14 

From  695  EB-  No.  1.     10/13/14. 

Temperature 

,  19.9°. 

Oct.  20,  14  young.     Distributed  to 

Time,  2  :  10  : 

00. 

695  G6-. 

1 

2:12:11,  + 

Oct.  23,  2d  brood. 

2 

12  :  15,  + 

Oct.  31,  preserved. 

3 

13  :  03,  + 

4 

13  :  05,  + 

695  F6-  No.  2. 

5 

13  :  27,  + 

Oct.  20,  13  young. 

6 

12  :  27,  + 

Oct.  23,  2d  brood. 

7 

14  :  00,  + 

Oct.  27,  discarded. 

8 

14  :  50,  -f 

9 

15  :  05,  -f 

695  F6-. 

10 

15  :  50,  + 

Oct.  20,  young. 

11 

18  :  00,  + 

Oct.  27,  discarded. 

12 

19:55,+ 

13 

over-time. 

14 

over-  time. 

1  However,  for  the  sake  of  complete  safety,  it  was  considered  wise  to  keep  up  the  three- 
entry  method  of  keeping  notes.  It  did  not  prove  unduly  laborious.  In  any  case,  a  new  sheet 
must  be  made  for  each  set  of  three  new  bottles  of  the  new  generation,  and  naturally  the  notes 
on  the  light  tests  required  the  entry  of  several  of  the  items  placed  in  the  experimental  note-book. 


A   PHYSIOLOGICAL   CHARACTER.  13 


METHOD  OF  SELECTION. 
GENERAL  PROCEDURE. 

Propagation  of  the  older  lines  of  D.  pulex  had  progressed  from 
5  to  8  generations  before  the  selection  experiments  were  begun. 
Selection  with  lines  later  used  was  begun  at  once  upon  introduction 
into  the  laboratory. 

The  culture-bottles  were  given  uniform  treatment  so  far  as 
possible.  The  plus  and  minus  strains  of  the  same  line  were  kept 
side  by  side  on  the  table.  The  only  obvious  environmental  differ- 
ences to  which  the  two  strains  of  the  same  line  were  subjected  arose 
from  the  fact  that  the  young  from  the  two  strains  were  transferred 
to  new  food,  usually  on  different  days.  While  every  effort  was  made 
to  provide  uniform  culture-water,  only  a  moderate  degree  of  uni- 
formity was  attainable,  but  this  unavoidable  difference  in  environ- 
mental treatment  was  not  differential,  and  in  the  long  run  should 
have  affected  the  two  strains  equally. 

Unlike  the  Cladocera  material  reared  under  laboratory  con- 
ditions by  many  workers,  the  writer's  material  produced  exclusively 
asexual  young,  there  being  with  the  material  subjected  to  selection 
no  discovered  case  in  which  males  or  sexual  eggs  were  produced. 

The  selection  tests  were  conducted  in  a  darkened  basement 
room  with  dull-black  walls.  The  tank  was  constructed  of  plate 
glass,  the  bottom  being  a  heavy  slab  of  smooth  slate,  grooved  out 
to  a  depth  of  about  1.5  cm.,  the  grooves  being  about  2  cm.  wide. 
The  plate-glass  sides  and  ends  were  set  in  these  grooves  in  an  aqua- 
rium cement.  The  grooves  were  filled  with  the  cement  so  as  to  leave 
a  smooth  surface  continuous  with  the  surface  of  the  slab.  The 
lines  of  contact  of  the  glass  sides  and  ends  were  cemented  together 
with  thick  Canada  balsam.  After  the  aquarium  cement  and  balsam 
had  been  given  several  days  for  hardening,  the  surface  of  the  cement 
and  the  inside  of  the  glass  sides  of  the  tank  were  coated  with  a  mix- 
ture of  lampblack,  balsam,  and  turpentine.  The  result  was  a  tank 
with  uniform  dull-black  sides  and  bottom  and  with  transparent  ends. 
The  tank  measured  inside  40  cm.  by  26.6  cm.  and  was  7.2  cm.  in 
depth.  An  improvement  on  this  tank  would  be  made  by  substituting 
thin  slabs  of  smooth  slate  for  the  plate-glass  sides. 

The  source  of  light  was  at  first  a  slender,  cylindrical,  carbon- 
filament,  incandescent  lamp.  Later  similar  carbon-filament  lamps 
could  not  be  obtained  and  a  tungsten  lamp  was  used.  The  lamps 
used  varied  from  43.5  to  75.9  candle-power.  They  were  placed  at 


14  SELECTION    IN    CLADOCERA    ON   THE    BASIS   OF 

the  appropriate  distances  from  the  tank  to  obtain  an  illumination 
of  120  candle-meters,  calculated  to  the  middle  of  the  tank.1 

The  heat-screen,  4.5  cm.  thick,  with  parallel  glass  sides  and 
filled  with  distilled  water,  was  placed  at  the  end  of  the  tank  through 
which  the  illumination  was  received.  An  upright  black  cardboard 
screen  was  placed  between  the  heat-screen  and  the  end  of  the  tank, 
so  as  to  permit  rays  of  light  to  enter  the  end  of  the  tank  only  in  the 
area  between  the  bottom  of  the  tank  and  the  level  of  the  surface 
of  the  water.  The  water  within  the  tank  was  filtered  pond-water 
and  was  maintained  at  a  depth  of  1.8  cm.  Fresh  water  was  placed 
in  the  tank  each  day  and  a  change  of  water  was  made  after  testing 
and  selecting  from  each  four  broods. 

The  young  animals  to  be  tested,  consisting  of  entire  first 
broods,  were  removed  from  the  mothers'  bottles  by  means  of  a  small 
pipette,  and  taken  to  the  experimental  room  in  10  mm.  cylindrical 
vials.  The  vials  were  placed  in  a  wire  basket  and  immersed  to 
about  3  cm.  depth  in  a  dish  of  water  of  the  same  temperature  as 
the  water  in  the  experimental  tank  and  left  about  an  hour  before 
the  tests  were  begun.  The  temperature  of  the  experimental  room 
varied  from  14°  C.  during  the  coldest  weather  in  winter  to  sometimes 
as  high  as  22°  C.  in  summer.  Very  rarely  the  temperature  was  as 
low  as  10°  C.  or  as  high  as  24°  C. 

In  preparation  for  the  testing  of  each  brood,  the  entire  brood 
was  (by  means  of  the  small  pipette)  placed  in  the  center  of  the 
experimental  tank  within  an  upright  cylinder  of  glass  1  cm.  in  diam- 
eter. The  experimental  light  was  turned  on  and  all  extraneous 
light  was  eliminated.  The  animals  were  then  left  undisturbed 
within  this  cylinder  for  2  minutes,  when  the  cylinder  was  lifted  care- 
fully from  the  water,  thus  releasing  the  animals  in  the  center  of  the 
tank;  the  temperature  of  water  and  time  of  release  of  the  animals 
were  recorded  just  before  the  release.  The  illumination  within  the 
water  was  sufficient  to  make  the  young  daphnids  just  visible  to  the 
eye  of  one  accustomed  to  working  with  them.  As  soon  as  a  daphnid 
reached  either  end  of  the  tank  it  was  removed  and  the  time  of  its 
arrival  recorded,  together  with  the  proper  sign  to  indicate  to  which 
end  of  the  tank  it  had  gone  (see  page  12).  The  interval  between  the 
release  of  the  animal  and  its  arrival  at  the  end  of  the  tank  is  referred 

1  No  allowance  was  made  for  the  diminution  in  amount  of  light  due  to  its  passage  through 
the  heat-screen,  the  plate-glass  end  of  the  tank,  and  the  water  within,  nor  was  allowance  made 
for  the  somewhat  different  results  in  illumination  due  to  using  lamps  of  different  candle- 
power.  While  the  distances  at  which  the  lamps  were  used  were  such  as  to  give  a  calculated 
illumination  of  120  candle-meters  at  the  middle  of  the  tank,  the  illumination  at  the  ends  of  the 
tank  would  of  course  be  somewhat  different  with  the  lamps  differing  in  candle-power.  This 
fact  was  not  taken  into  consideration  in  originally  substituting  a  60-watt  Mazda  lamp  (about 
75.3  candle-power)  for  the  carbon-filament  lamp  of  43.5  candle-power.  Having  for  a  time  used 
the  lamp  with  the  higher  candle-power,  it  was  considered  wise  to  continue  its  use.  Any  variation 
in  absolute  intensity  of  the  light  at  the  different  parts  of  the  tank  is  regrettable,  but  it  should 
of  course  be  equally  effective  with  the  plus  and  minus  strains  and  presumably  could  not  disturb 
the  course  of  the  experiments. 


A   PHYSIOLOGICAL   CHARACTER.  15 

to  as  its  reaction-time.1  The  test  was  continued  for  15  minutes, 
unless  all  individuals  had  reached  an  end  of  the  tank  earlier;  any 
remaining  at  the  end  of  15  minutes  were  removed  and  arbitrarily 
assigned  a  reaction-time  of  15  minutes.2  Another  brood  was  then 
tested  in  a  like  manner. 

In  testing  a  brood  of  the  plus  strain,  the  individual  first  reaching 
the  positive  end  of  the  tank  was  at  once  placed  in  a  separate  vial 
and  later  transferred  to  the  No.  1  bottle  of  the  new  generation.  The 
second  one  to  reach  the  positive  end  was  likewise  placed  in  a  separate 
vial,  to  be  later  transferred  to  the  No.  2  bottle.  The  others  were 
returned  to  the  vial  in  which  they  had  been  conveyed  to  the  experi- 
mental room.  Three  or  more  of  these  were  later  transferred  to  the 
' '  prime  "  bottle  of  the  new  generation.  With  the  broods  of  the  minus 
strains  the  procedure  was  the  same,  except  that  the  two  quickest  to 
reach  the  negative  end,  the  two  which  moved  farthest  toward  the 
negative  end,  the  two  moving  least  toward  the  positive  end  of  the 
tank,  or  the  two  slowest  in  reaching  the  positive  end  were  selected 
for  the  No.  1  and  No.  2  bottles  of  the  new  generation.  Negatively 
reacting  individuals  did  not  occur  in  most  of  the  broods,  nor  were 
there  usually  individuals  which  showed  any  tendency  to  react  nega- 
tively to  light.  With  D.  pulex  and  D.  longispina  few  individuals 
failed  to  move  to  the  positive  end  within  the  limits  of  the  experiment. 
Hence  very  frequently  in  the  minus  strains  the  No.  1  and  No.  2 
individuals  selected  were  respectively  the  one  slowest  and  the  one 
next  slowest  in  reaching  the  positive  end  of  the  tank. 

As  with  the  data  for  the  over-time  individuals,  the  data  for  the 
negatively  reacting  individuals  presented  some  difficulty.  The 
occurrence  of  negatively  reacting  individuals  was  irregular  and  more 
or  less  spasmodic.  Table  2  presents  some  data  illustrating  this 
point.  Reference  to  any  of  the  tables  presenting  the  data  by  broods 
will  show  that  when  negatively  reacting  individuals  occur  they  are 
frequently  relatively  numerous.  The  writer  believes  negatively  re- 
acting individuals  are  (usually,  at  any  rate)  influenced  by  some 
unusual  environmental  factor  and  that  an  individual's  swimming 

xThe  "reaction-time"  as  recorded  in  the  notes  and  used  in  this  paper  indicates,  in  seconds, 
the  interval  between  the  release  of  the  young  daphnid  from  the  glass  cylinder  in  the  middle  of 
the  experimental  tank  and  the  time  at  which  it  reached  the  end  of  the  tank.  Strictly  speaking, 
this  is  the  real  reaction-time — the  time  consumed  in  the  beginning  of  movement  and  estab- 
lishing orientation  with  reference  to  the  light — plus  the  time  consumed  by  the  animal  in  swimming 
to  the  end  of  the  tank. 

2  The  arbitrary  assumption  of  15  minutes  as  the  reaction-time  of  individuals  which  as  a 
matter  of  fact  did  not  react  within  that  length  of  time  is  open  to  criticism,  but  no  better  method 
of  utilizing  this  significant  portion  of  the  data  was  ascertained.  The  "  over-time "  individuals 
can  not  be  disregarded.  The  data  for  them  are  obviously  very  significant.  In  a  slightly  reactive 
strain,  such  as  the  minus  strain  of  Line  757,  they  constitute  a  large  portion  of  the  individuals 
tested  and  represent  a  striking  manifestation  of  the  low  responsiveness  to  light  in  this  as  in  other 
strains.  As  pointed  out  in  another  connection,  the  arbitrary  assumption  of  900  seconds  for 
their  reaction-times  greatly  minimizes  the  rightful  influence  upon  mean  reaction-time  of 
their  slight  or  non-reactiveness  to  light. 


16  SELECTION    IN    CLADOCERA   ON   THE    BASIS   OF 

to  the  negative  end  of  the  tank  does  not  in  most  cases  really  indicate 
a  negative  phototropism.  Such  individuals,  if  tested  a  second  time, 
do  not  ordinarily  repeat  the  behavior.1  The  best  interpretation 
seemed  to  be  that  individuals  which  reacted  negatively  were  stimu- 
lated by  the  light  as  their  sisters  were,  but  that  a  " negative"  re- 
sponse was  ordinarily  called  forth  by  some  accessory  influence. 
Occasionally  this  additional  influence  obviously  was  accidental 
mechanical  stimulation;  but  usually  negative  responses  could  not  be 
thus  accounted  for,  and  often  they  were  certainly  not  due  to  this 
cause. 

The  data  for  negatively  reacting  individuals  were  treated  as 
though  the  individuals  had  reacted  positively,  except  that  the  number 
of  such  occurrences  was  tabulated  and  is  frequently  referred  to  in  the 
treatment  of  the  data.  It  is  questionable  if,  in  making  selections 
in  the  minus  strains,  one  was  justified  in  selecting  individuals  (where 
such  a  selection  was  possible)  which  went  to  the  negative  end  of  the 
tank.  Cladocera  are  so  generally  positive  that  negative  reactions 
are  at  once  open  to  a  question  as  to  significance,  but  it  is  believed 
that  the  possibility  of  an  actual  change  or  mutation  producing  a 
really  negatively  reacting  individual  is  sufficient  justification  for 
selecting  these  individuals  in  the  minus  strains. 

In  case  a  daphnid  remained  unaccounted  for  at  the  end  of  the 
test  of  a  brood,  the  tank  was  emptied,  twice  carefully  rinsed,  and 
filled  to  the  proper  depth  with  fresh  water.  At  all  times  in  the 
experiments  every  conceived  precaution  was  taken  to  exclude 
extraneous  light,  to  guard  against  reflection  from  any  object  without 
or  small  foreign  body  within  the  tank,  to  eliminate  all  mechanical 
stimulation,  and  particularly  to  guard  against  the  possible  mis- 
placing of  an  individual  daphnid  in  the  handling  of  the  broods.2 

In  brief,  in  the  selection  experiments  the  entire  first  brood,  soon 
after  its  release  from  the  mother's  brood-pouch,  was  removed  from 
the  culture-bottle,  placed  in  a  small  vial,  taken  to  a  darkened 
room,  and  subjected  to  known  and  always  uniform  directive  light- 
stimulation  under  carefully  controlled  conditions,  the  object  being 
to  attempt  to  procure  (by  selection  through  many  generations)  a 
strain  more  responsive  to  light  than  a  second  strain  from  the  same 

1  In  the  plus  strain  of  Line  757  minus  individuals  occurred  in  17  broods  out  of  172  broods 
from  which  selections  were  made.    In  4  of  these  cases  there  were  more  than  a  single  minus  indi- 
vidual (19  individuals  in  4  broods).    In  the  minus  strain  of  Line  757  minus  individuals  occurred 
in  13  of  175  actual  selection  tests,  and  in  one  of  these  cases  there  were  3  minus  individuals 
(table  42). 

2  The  pipette  in  which  the  daphnids  were  handled  was  carefully  rinsed  after  handling  each 
brood.    In  conducting  the  selections  the  vials  to  receive  the  selected  animals  were  arranged  so 
that  the  one  nearest  at  hand  was  the  one  in  which  the  next  individual  removed  from  the  tank 
was  to  be  placed.    A  small  opening  was  made  in  the  cardboard  screen  in  order  to  permit  a  small 
amount  of  light  to  fall  upon  the  observer's  watch  and  note-book.    The  pipette  could  readily  be 
examined  in  this  light  and  the  animal  seen  within  it,  in  case  there  was  any  doubt  about  the 
daphnid  having  been  drawn  into  the  pipette  when  its  removal  from  the  tank  was  attempted. 
The  table  on  which  the  experiments  were  conducted  was  closed  off  from  the  remainder  of  the 
room  by  a  black  curtain. 


A   PHYSIOLOGICAL   CHARACTER.  17 

line  selected  for  its  less  responsiveness  to  light.  In  the  beginning  of 
this  selection  the  most  responsive  individual  from  the  first  brood  of 
the  young  mother  was  selected  for  the  beginning  of  the  plus  strain 
and  the  least  reactive  one  or,  perchance,  one  reacting  negatively  and 
avoiding  the  light,  for  the  beginning  of  the  minus  strain.  In  the 
next  and  later  generations  selections  were  made  from  the  first  broods 
(except  in  cases  where  the  first  brood  was  lost),  the  two  most  reactive 
individuals  in  the  plus  and  the  two  least  reactive  individuals  (or 
negatively  reacting  individuals  if  such  occurred)  in  the  minus  strain 
being  selected  to  propagate  and  continue  their  respective  strains. 

At  certain  periods  during  the  course  of  these  experiments  the 
rearing  of  the  various  strains  was  rendered  difficult  by  poor  food 
conditions.  Sometimes  during  such  periods  selections  were  not 
made  on  the  basis  of  reaction  to  light,  but  the  individuals  used 
to  continue  the  strains  were  taken  at  random  from  the  mother's 
bottle.  In  the  tables  giving  the  data  by  broods,  such  cases  (and  a 
few  others  in  which  for  some  reason  the  selection  test  was  not  con- 
ducted) are  indicated  as  " random  distributions." 

ENVIRONMENTAL  INFLUENCES  AND  REACTION-TIME. 

The  tables  of  data  obtained  in  making  the  selections  show  that 
there  were  wide  differences  in  the  general  reactiveness  of  different 
broods  tested  in  making  the  selections  (see  tables  3  and  4).  Some- 
times a  brood  had  an  unusually  low  or  an  unusually  high  mean  re- 
action-time, when  perhaps  the  immediately  preceding  and  next 
succeeding  broods  of  the  same  strain  had  a  mean  near  that  for  the 
strain  as  a  whole.  Sometimes  a  considerable  number  of  a  single  brood 
reacted  negatively,  although  on  the  whole  only  a  small  percentage  of 
the  individuals  reacted  negatively.  Occasionally  it  was  noted  that 
all  the  broods  on  a  certain  day  responded  slowly  to  light  stimulation, 
while  perhaps  on  the  following  day  all  responded  promptly.  These 
differences  are  due  to  environmental  factors.  Table  2  illustrates 
this  point,  as  well  as  the  spasmodic  occurrence  of  negatively  reacting 
individuals.  Reference  to  this  table  shows  that  on  August  29,  1913, 
the  mean  reaction-time  of  87  individuals  of  Line  695  tested  was 
401  seconds,  and  3.5  per  cent  reacted  negatively;  on  the  following 
day  the  mean  reaction- time  was  636  seconds  and  1.6  per  cent  were 
negative;  on  the  day  following  this  the  mean  reaction-time  was 
398  seconds  and  28  per  cent  were  negative.  Thus,  on  the  second 
of  these  three  days  the  individuals  of  the  same  strains  were  slower  in 
their  reactions,  compared  with  the  other  two  days,  by  approximately 
60  per  cent.  And  there  were  twice  and  18  times  as  large  percentages 
of  negatively  reacting  individuals  on  the  first  and  third  as  on  the 
second  of  these  days.1 

1  This  is  an  extreme  case,  but  it   illustrates  differences  such  as  were  repeatedly  observed, 
though  usually  to  a  much  less  marked  degree. 


18 


SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 


Such  differences  in  response  indicate  environmental  factors  of 
great  influence,  but  what  these  influences  were  was  in  most  cases 
not  determined.  It  was  frequently  noted,  in  the  tests  at  the  begin- 
ning of  these  experiments,  that  after  the  observer  had  tested  several 
broods  in  succession  the  broods  tested  later  reacted  less  quickly  than 
the  earlier  ones.  This  may  have  been  due  to  the  accumulation  in 
the  water  of  C02  from  the  observer's  breath.  The  difficulty  was 
obviated  by  changing  the  water  in  the  experimental  tank  after  testing 
each  four  broods. 

These  and  other  factors  were  much  too  influential  for  one  to 
ignore  them  completely,  but  their  influence  was  presumably  a 
horizontal  one,  affecting  all  the  individuals  of  a  brood  and  of  the 
different  strains  in  the  same  manner  and  not  in  any  way  influencing 
selections,  except  as  affecting  the  reaction-foVie  and  in  causing  a  few 

TABLE  2. — Data  from  an  early  test  series  of  Line  695  to  illustrate  differences  in  general 
reactiveness  to  light  on  successive  days. 


Mother's  pedigree. 

No. 
of 
young. 

No.  of  nega- 
tively react- 

individuals. 

Percentage 
of  negative 
individuals. 

Mean  reaction-time  in 
seconds. 

By  broods. 

By  days. 

Aug.  29,  1913. 
Fresh  water 

695  E3  -  No.  9'.... 
695  I3  +  No.  5  
695  Is  4-  No.  6  

Totals,  etc  

695  Is  +  No.    9  .  . 
695  E3  -  No.    2  
695  E3  -  No.  10  
695  I3  +  No.  12  
695  E3  -  No.    7.  . 
695  Is  +  No.  11  
695  I3  +  No.  13  
695  Es  -No.    4  

Totals,  etc  

695  Is  +  No.    1  .  . 
695  Es  -  No.    9  
695  I3  +  No.    8  

Totals,  etc  

26 
23 
38 

1 
0 
2 

448 
555 
276 

Fresh  water 

Aug.  30,  1913. 
Fresh  water  

Fresh  water  

87 

3 

3.5 

401 

22 
21 
23 
23 
27 
33 
21 
20 

0 
0 
0 
0 
1 
2 
0 
0 

346 
631 

704 
710 
577 
766 
761 
526 

Aug.  31,  1913 
Fresh  water 

190 

3 

1.6 

636 

22 
17 
11 

10 
2 
2 

350 
396 
498 

50 

14 

28.0 

398 

'This  is  mother  No.  9  of  the  fifty-seventh  generation  of  the  minus  strain  of  Line  695. 

individuals  which  otherwise  would  have  been  positive  to  be  rendered 
negative  in  their  reactions.  It  seems  wise  to  postpone  the  detailed 
analysis  of  the  effects  of  environment  upon  reaction-time  and  upon 
vigor  until  after  the  general  analysis  of  the  data  has  been  presented 
(see  pages  128-149). 

TEST  SERIES. 

Because  of  the  various  factors  instrumental  in  influencing  the 
reactions  of  the  different  broods  of  Cladocera,  and  in  order  to  obtain 
a  critical  measure  of  the  effect  of  selection,  if  such  occurred,  so-called 


A   PHYSIOLOGICAL   CHARACTER.  19 

test  series  were  planned.  These  test  series  were  conducted  under  a 
plan  by  which  large  numbers  of  broods  of  the  plus  and  the  minus 
strains  of  the  same  line  were  experimented  upon  on  the  same  day 
and  as  nearly  as  was  experimentally  possible  under  precisely  the  same 
conditions.  The  plan  was  as  follows:  First,  it  was  necessary  to 
wait  until  such  time  as  the  plus  and  minus  strains  of  the  same  line 
reproduced  on  the  same  day.  A  number  of  these  young,  from  12 
to  20,  of  each  of  the  strains  were  then  transferred  to  individual 
bottles,  given  the  same  food,  kept  grouped  together  on  the  same 
table,  and  in  every  way  treated  alike.  When  their  first  broods 
appeared,  four  broods  of  equal  or  nearly  equal  size — two  from  the  plus 
and  two  from  the  minus  strain — were  chosen,  the  choice  being  limited 
to  broods  released  from  the  mother's  brood-pouch  within  2  or  3  hours 
of  the  same  time.  Thus  we  had  broods  containing  equal  or  nearly 
equal  numbers  of  individuals,  of  the  same  age  (within  2  or  3  hours), 
and  from  mothers  of  the  same  age  which  had  themselves  received 
identical  treatment  from  birth;  other  " quartettes"  were  selected  in 
like  manner.  Further,  the  four  broods,  constituting  a  quartette, 
were  handled  in  a  definite  order;  a  plus  brood  was  experimented  with 
first,  then  a  minus  brood,  then  the  second  minus  brood  of  the  quar- 
tette, and  finally  the  second  plus  brood  was  tested.  The  tank  was 
then  emptied  and  replenished  with  fresh  water.  The  second  quar- 
tette was  handled  in  reverse  order:  first  a  minus  brood,  then  (in 
order)  the  two  plus  broods,  and  the  second  minus  brood.  The  next 
quartette  was  then  handled  in  the  order  indicated  for  the  first  quar- 
tette. Still  other  quartettes  of  broods  were  chosen  from  the  second 
and  later  broods  of  the  test-series  mothers  until  the  numbers  of  in- 
dividuals tested  from  each  strain  were  quite  large — in  most  cases 
larger  than  600  and  in  several  cases  in  excess  of  1,000. 

The  test-series,  it  was  hoped,  might  serve  as  a  means  of  eliminat- 
ing most  of  the  disturbing  factors  unquestionably  present  during  the 
reaction-tests,  due  to  the  fact  that  the  selections  in  the  plus  strain  and 
the  minus  strain  of  the  same  line  were  usually,  through  necessity, 
made  on  different  days. 

SAME-DAY  BROODS. 

The  two  strains  of  the  same  line  only  occasionally  reproduced 
on  the  same  day,  so  that  consecutive  reaction-tests  of  a  brood  of 
each  of  these  two  strains  were  not  ordinarily  possible;  however, 
during  the  whole  course  of  the  selection  experiments,  there  were  a 
number  of  selections  in  the  plus  and  minus  strains  of  the  same  line 
on  the  same  days.  In  addition  to  the  complete  tabulation,  the  data 
for  these  " same-day"  broods  are  tabulated  separately.  Such  a 
tabulation  for  the  two  strains  of  Line  695  is  given  in  table  8.  Such 
a  comparison  of  the  mean  reaction-times  of  the  same-day  broods 
ought  perhaps  to  afford  a  safer  criterion  of  the  effectiveness  of 


20        SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

selection  (if  an  effect  be  obtained)  than  a  summary  of  the  entire 
selection  data.  The  difficulty  is  that  the  amount  of  this  data  is  so 
limited.  For  example,  there  are  only  41  of  these  same-day  broods 
for  Line  695,  out  of  a  total  of  192  broods  of  the  plus  strain  and  180 
broods  of  the  minus  strain  for  which  data  were  secured. 

POSSIBLE  CRITICISMS  OF  METHODS  OF  SELECTION. 

Certain  possible  criticisms  of  the  methods  used  in  the  selection 
experiments  occurred  to  the  writer  and  doubtless  may  occur  to  others. 

1.  Was  there  assurance  that  the  individual  daphnids  selected 
were  really  the  most  and  the  least  positively  reacting  individuals  in 
their  respective  broods?     (a)  Was  there  reason  to  think  that  the 
individual  daphnid  first  reaching  the  positive  end  of  the  tank  was  the 
one  most  influenced  by  the  light?    (6)  May  not  this  individual  have 
been  affected  by  a  slight  swirl  in  the  water  in  being  released  from 
the  cylinder  or  in  some  other  manner  subjected  to  mechanical  stimu- 
lation?   (c)  Or  may  it  have  been  affected  differently  from  its  fellows, 
due  to  some  other  condition  external  to  itself?    (d)  Or  may  it  have 
been  influenced  by  some  internal  factor  or  factors  and  consequently 
its  reaction  determined  by  some  " physiological  state?"    (e)  Or  may 
its  reaction-time  not  have  been  influenced  by  its  general  vigor  or  by 
its  ability  as  a  swimmer?    (/)  Was  the  method  of  handling  such  as 
in  any  manner  to  unfavorably  affect  the  animals  and  thus  render 
their  activities  untrustworthy? 

2.  If  the  individual  selected  was  really  the  most  positively  photo- 
tactic  at  the  time  of  the  selection,  was  it  also  the  most  positively 
phototactic  an  hour  or  a  day  later? 

3.  Was  the  selection  method  used  calculated  to  produce  a  more 
vigorous  race  as  compared  with  a  less  vigorous  race? 

4.  Assuming  real  and  lasting  differences  in  light-reaction  be- 
tween sibs,  are  these  differences  heritable  anyway? 

If  one  accepted  these  possible  criticisms  in  their  fullest  impli- 
cation it  would  at  once  be  seen  that  the  results  here  set  forth  were 
impossible  of  attainment,  and  the  result  with  Line  757  is  perhaps 
sufficient  answer  to  these  questions.  The  writer  does  not  believe 
that  these  possible  criticisms  are  at  all  damaging  to  or  can  be  right- 
fully applied  to  these  experiments;  however,  they  will  be  discussed 
individually.  (1)  and  (a)  under  it  will  be  discussed  after  its  sub- 
sidiary questions,  (b)  to  (/),  have  been  considered. 

(6)  Observations  directed  upon  the  effect  of  mechanical  stimu- 
lation show  that  a  daphnid  when  mechanically  stimulated  usually 
moves  promptly  and  rapidly;  frequently  the  movement  is  from  the 
source  of  mechanical  stimulation  if  that  is  directive;  but  if  non- 
directive,  the  movement  is  equally  prompt  and  vigorous,  though 
usually  not  different  in  direction  from  that  in  which  the  animal  was 


A   PHYSIOLOGICAL   CHARACTER.  21 

previously  oriented.     In  either  case  the  effect   is   ordinarily  very 
temporary. 

In  the  vast  majority  of  cases,  in  making  the  selections,  when  the 
cylinder  was  lifted  to  release  the  animals  the  daphnids  at  first  failed 
to  move  at  all  or  moved  very  slowly  and  only  later  began  swimming 
or  increased  the  rate  of  movement.  In  spite  of  all  precautions, 
doubtless  some  individuals  were  mechanically  stimulated  and  thus 
influenced  in  their  reactions;  occasionally  such  was  obviously  the 
case.  But  while  such  stimulation  might  influence  the  rate  of  move- 
ment and  occasionally  the  direction  of  movement,  these  effects  were 
temporary  and  the  occurrences  of  such  influences  were  to  all  appear- 
ances infrequent,  so  that  it  is  believed  the  aggregate  effect  of  such 
influences  upon  the  selections  is  of  little  or  no  significance. 

(c)  Unquestionably  it  is  true  that  even  when  every  precaution 
is  taken  to  secure  uniformity  of  conditions  and   to  eliminate  all 
extraneous  light,  to  eliminate  disturbances  of  the  direction  of  the 
incident  rays  of  light,   to  prevent  mechanical  or  other  accessory 
stimulation,  and  to  provide  uniformity  in  handling  the  animals,  etc., 
perfect  conditions  are  not  attainable.    Imperfect  experimental  con- 
trol of  environmental  conditions  is  obvious  enough  and  is  referred  to 
elsewhere  in  this  paper  (see  pages  15, 17-20,  and  128-149).    But  there 
are  no  grounds  for  assuming  that  these  imperfectly  controlled  environ- 
mental conditions  were  differential  or  in  any  appreciable  way  affected  the 
selections.    To  assume  that  in  any  considerable  number  of  the  selec- 
tions differential  stimulation  occurred  would,  we  believe,  go  beyond 
the  facts  and  be  quite  unwarranted. 

(d)  It  is  obvious  that  conditions  internal  to  the  animals  them- 
selves are  not  subject  to  complete  control.     The  writer  does  not 
doubt  that  " physiological  states''  were  occasionally  a  factor,  and 
sometimes  a  determining  factor,  in  individual  selections,  but  he  does 
not  believe  that  internal  conditions  were  frequently  a  determining 
factor.    In  cases  of  repeated  tests  of  the  reaction-times  of  a  number 
of  individual  daphnids,  sometimes  several  hours  or  even  2  or  3  days 
apart,  the  later  results  usually  agreed  with  the  earlier  results — i.  e., 
the  animals  which  at  the  first  test  were  most  responsive  to  the  light 
on  subsequent  tests  were  the  most  responsive  or  among  the  most 
responsive  individuals.    Hence  there  is  excellent  reason  for  believing 
that  the  individuals  selected  were  actually  the  most  responsive  and 
the  least  responsive  individuals  of  their  respective  broods.     But 
assuming  that  temporary  physiological  states  did  sometimes  influence 
the  selections  (as  must  occasionally  have  been  the  case),  it  would 
merely  operate  to  render  the  selections  less  effective,  and  unless  the 
internal  temporary  physiological  factors  were  factors  in  a  prevailing 
number  of  the  selections,  they  could  not  neutralize  the  selections. 

(e)  The  individuals  selected  in  the  plus  strains  may  have  been 
merely  the  best  swimmers,  regardless  of  general  vigor,  or  they  may 


22        SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

have  been  the  most  vigorous  individuals  of  their  respective  broods; 
but  on  the  whole  there  seems  no  good  reason  for  believing  this  to 
have  been  the  case.  If  the  differences  were  merely  differences  in 
quickness  of  movement  or  swimming  ability,  it  would  be  quite  as 
interesting  to  develop  "hare"  and  "tortoise"  strains  of  Cladocera  as 
to  develop  strains  more  reactive  and  less  reactive  to  light;  and  the 
result  would  have  the  same  fundamental  bearing  on  the  problem  of 
selection  within  the  pure  line.  That  the  reaction-times  of  the  vast 
majority  of  individuals  were  not  influenced  by  differences  in  vigor  is 
attested  by  the  fact  that  there  is  no  consistent  relation  between 
vigor  (as  measured  by  the  mean  reproductive  index)  and  mean  re- 
action-time. This  point  is  discussed  more  fully  in  connection  with 
the  detailed  analysis  of  the  data  for  the  various  lines. 

(/)  The  strains  subjected  to  selection  did  not  show  greater 
mortality  nor  less  vigorous  growth  and  reproductive  ability  than 
those  lines  not  used  in  the  selection  experiments.  The  only  difference 
in  conditions  through  which  those  selected  (as  compared  with  those 
strains  not  subjected  to  selection)  passed  previous  to  being  placed 
in  the  experimental  tank  was  being  placed  for  a  time  in  a  relatively 
small  amount  of  the  culture-water.  There  is  no  reason  to  think  that 
this  profoundly  affected  their  light-reactions,  or  if  it  had,  that  the 
effect  would  not  have  been  a  horizontal  rather  than  a  differential  one. 

(1)  and  (la)  On  numerous  occasions  the  individuals  of  a  brood 
were  caused  to  react  in  the  experimental  tank  a  number  of  times. 
Of  course,  on  second  and  subsequent  trials  only  a  single  individual 
could  be  handled  at  one  time  in  the  tank  because  of  the  danger  of 
mistaking  the  identity  of  the  individuals. 

Of  10  such  experiments,  conducted  within  a  few  days  and  includ- 
ing all  such  experiments  made  within  those  days,  there  was  com- 
plete agreement  in  reaction  order  in  6  experiments;  in  3  there  was 
fair  agreement,  and  in  1  there  was  very  poor  agreement  in  reaction 
order.  Even  in  the  last-mentioned  case,  however,  the  individuals 
which  reacted  more  quickly  during  the  first  test  in  general  reacted 
more  quickly  on  the  average  of  all  the  trials,  but  the  reaction  order 
was  greatly  changed.  The  objection  that  the  selections  in  general 
may  have  been  made  on  the  basis  of  vigor  of  individuals  has  been, 
we  think,  effectively  discounted.  In  view  of  this  fact,  it  is  believed 
that  such  consistent  results  as  those  just  indicated  show  conclusively 
that  neither  physiological  states  nor  any  other  factor  operated  to 
obviate  or  hamper  the  selection  on  the  basis  of  fundamental  indi- 
vidual differences  in  reactiveness  to  light.  It  is  believed  that  in  the 
vast  majority  of  cases  the  two  selected  in  the  plus  strains  were  more 
reactive  to  the  illumination,  to  which  they  were  subjected  than  their 
fellows,  and  that  those  selected  in  the  minus  strain  were  less  positive 
than  their  fellows. 


A   PHYSIOLOGICAL   CHARACTER.  23 

2.  This  point  bears  on  the  question  of  the  occurrence  of  "  physio- 
logical states"  and  their  relative  frequency.    The  discussion  of  (1) 
and  (la)  bears  directly  on  this  point. 

3.  To  the  possible  objection  that  the  method  of  selection  was 
suited  to  the  production  of  more  vigorous  strains  in  the  plus  and  less 
vigorous  strains  in  the  minus  strains,  the  answer  is  that  such  a  result 
did  occur  to  some  extent  in  two  of  the  lines,  but  it  was  not  a  cumu- 
lative result  and   the  differences  in  reproductive  vigor  were  not 
greater  at  the  close  than  during  the  early  part  of  the  experiment. 
These  cases  are  discussed  in  detail  in  the  analysis  of  the  data  for  the 
various  lines.     But,  as  is  pointed  out  later,  in  those  cases  in  which 
reduced  reproductive  vigor  did  occur  it  was  not  causally  associated  with 
differences  in  reaction-time. 

4.  This  question,   as  applied  to  the  selection  problem,  is  a 
" leading  "  question.    Its  answer  depends  upon  whether  the  differences 
are  germinal  or  purely  somatic.    In  the  vast  majority  of  cases  such 
reaction  differences  as  are  encountered  in  these  selection  experiments 
are  probably  purely  somatic,  and  the  effect  of  selecting  such  variants 
is  of  course  nil.    Whether  some  of  these  reaction  differences  are  due 
to  germinal  modifications  will  be  taken  up  in  connection  with  the 
experimental  data  considered  in  this  publication. 


24  SELECTION    IN    CLADOCERA    ON    THE    BASIS    OF 

NATURE  OF  SWIMMING  MOVEMENTS  OF  CLADOCERA 
SPECIES  USED  IN  SELECTION. 

Daphnia  pulex  is  rather  rotund  in  form  and  is  only  slightly  heavier 
than  water.  It  swims  by  vigorous  strokes  with  perceptible  intervals 
between,  thus  producing  a  very  jerky  movement.  It  is  pelagic  in 
its  habits,  and  ordinarily  its  swimming  strokes  are  repeated  just 
frequently  enough  to  maintain  its  level  in  the  water.  When  loco- 
motion occurs  the  animal  orients  itself  differently,  i.  e.,  instead  of  its 
antero-dorsal  axis  being  approximately  vertical,  it  changes  to  ap- 
proach a  horizontal  position  with  dorsal  side  uppermost;  its  swimming 
movements  are  more  rapidly  repeated  and  the  animal's  course  may  be 
fairly  straight.  The  jerky  character  of  movement  is  still  quite  in 
evidence.  This  species  frequently  attaches  to  the  surface  film  of 
the  water,  but  almost  never  to  the  sides  of  the  container.  A  vigorous 
individual  rarely  or  never  rests  upon  the  bottom. 

D.  longispina  is  very  similar  to  D.  pulex  in  its  habits  and  move- 
ments, but  is  more  slender  and  somewhat  less  active;  it  is  not  quite 
so  heavy;  its  axis  does  not  so  nearly  assume  a  vertical  position  in  its 
" place-maintaining "  movements;  and  in  its  locomotion  its  move- 
ments are  somewhat  less  jerky,  due  to  its  relatively  smaller  antennae 
and  less  vigorous  strokes.  Like  D.  pulex,  it  is  distinctly  pelagic  and 
does  not  rest  upon  the  bottom.  It  does  not  ordinarily  attach  to  the 
surface  film  and  never  to  the  sides  of  a  container. 

Simocephalus  exspinosus  is  a  rotund,  rather  bulky  species,  much 
heavier  than  the  two  species  of  Daphnia.  It  settles  rather  rapidly 
through  the  water,  unless  kept  up  by  continued  swimming  movements. 
It  lies  upon  and  moves  about  over  the  bottom  much  of  the  time,  fre- 
quently holds  fast  to  the  side  of  the  container,  and  often  attaches  to 
and  moves  about  at  the  surface  film.  It  usually  swims  with  ventral 
side  uppermost.  Its  swimming  movements  are  less  jerky  than  in 
either  species  of  Daphnia,  due  to  its  relatively  smaller  swimming 
antennae  and  more  rapid  antennal  movements,  though  the  jerky 
character  is  somewhat  evident.  In  nature  S.  exspinosus  spends  most 
of  its  time  upon  the  bottom  and  upon  submerged  aquatic  vegetation. 

BEHAVIOR  OF  YOUNG  OF  DIFFERENT  SPECIES  WHEN 
RELEASED  IN  THE  EXPERIMENTAL  TANK. 

In  the  experimental  tank  the  vast  majority  of  individuals  of  D. 
pulex  upon  their  release  swam  a  few  strokes  without  orientation, 
then  oriented  with  reference  to  the  light  and  swam  toward  the  source 
of  light.  This  orientation  was  not  perfect  in  many  cases,  so  that  the 
animal  sometimes  swam  somewhat  diagonally.  But  even  when  the 
direction  of  movement  was  somewhat  diagonal,  the  course  was 
usually  fairly  straight.  This  diagonal  course  occasionally  led  to  the 
side  of  the  tank.  In  such  cases  the  animal  usually  followed  along 


A   PHYSIOLOGICAL  CHARACTER.  25 

the  side  of  the  tank  in  the  direction  of  its  former  orientation,  though 
sometimes  it  took  a  diagonal  course  away  from  the  side  of  the  tank, 
but  in  the  general  direction  (as  regards  light)  in  which  it  was  formerly 
oriented.  The  individuals  which  were  slow  in  reacting  either  moved 
very  slowly,  scarcely  more  than  executing  "  place-maintaining  move- 
ments/7 or  they  wandered  about  independently  of  the  directive 
light  stimulation  for  a  time  and  later  attained  and  maintained  orien_ 
tation  with  reference  to  the  light.  Some  individuals  wandered  about 
so  long  or  swam  so  little  that  they  did  not  reach  either  end  of  the 
tank  in  the  15  minutes  of  the  experiment.  These  were  the  "  over- 
time" individuals  which  were  arbitrarily  assigned  a  reaction- time  of 
900  seconds.  D.  pulex  seldom  attached  to  the  surface  film  or  to  the 
sides  of  the  tank.  Negatively  reacting  individuals  were  not  especially 
common  with  D.  pulex. 

The  statements  regarding  the  behavior  of  D.  pulex  in  the  experi- 
mental tank  apply  to  D.  longispina;  this  species  sometimes  became 
caught  in  the  surface  film  and  was  unable  to  free  itself.  Such  indi- 
viduals were  discarded  and  excluded  from  the  record. 

S.  exspinosus,  on  being  released  within  the  experimental  tank, 
settled  to  the  bottom  at  once  or  swam  a  few  strokes  without  orien- 
tation and  then  settled  to  the  bottom.  Occasionally  some  individuals 
just  after  their  release  swam  upward  and  attached  to  the  surface 
film.  In  many  broods,  some  individuals,  after  settling  to  the  bottom 
or  rising  to  the  surface,  made  no  further  movements  during  the  15 
minutes  of  the  test,  while  with  some  broods  there  was  no  further 
movement  on  the  part  of  any  individual.  The  majority  of  individuals, 
however,  began  moving  again  within  a  few  seconds,  or  at  most  within 
2  or  3  minutes,  assumed  their  orientation,  and  swam  toward  the 
light.  Most  individuals,  after  orienting,  paused  for  a  time,  settled 
to  the  bottom,  and  afterward  continued  their  movement  toward  the 
light;  others,  after  the  second  pause,  did  not  move  again.  With  S. 
exspinosus  there  was  less  random  movement  after  the  first  pause  than 
with  the  two  species  of  Daphnia.  Occasionally  individuals,  in  their 
movement  toward  the  light,  swam  near  the  surface,  and  when  they 
paused  attached  to  the  surface  film.  Such  individuals  frequently  did 
not  move  again.  As  with  Daphnia,  the  orientation  often  was  not 
perfect  and  some  individuals  reached  the  sides  of  the  tank,  fre- 
quently attaching  there,  and  making  no  further  movements;  but  more 
generally  they  followed  along  the  side  of  the  tank  with  the  same 
general  orientation  as  before.  Orientation  in  a  negative  way  was 
uncommon  with  this  species. 

The  frequency  of  occurrence  of  large  numbers  of  non-reactive 
individuals  of  S.  exspinosus  was  a  distinct  difficulty  to  satisfactory 
experimental  conditions.  Because  of  this  apparent  difficulty,  some 
of  the  earlier  lines  of  S.  exspinosus  with  which  selection  was  attempted 


26  SELECTION   IN    CLADOCERA   ON   THE   BASIS    OF 

were  discarded  as  apparently  unprofitable  material  for  a  selection 
experiment  on  the  basis  of  reactiveness  to  light.  The  lines  of  S.  ex- 
spinosus which  were  retained  did  not  contain  fewer  non-reactive 
individuals  during  their  early  history,  but  the  experiments  were 
continued,  and  the  results  justify  the  conclusion  that  there  was 
after  all  a  sufficient  basis  for  an  experiment  in  selection. 

RELATION  BETWEEN  ENVIRONMENTAL  CONDITIONS  AND 
REPRODUCTIVE  AGE. 

Reproductive  age  is  very  closely  related  to  environmental  con- 
ditions, principally  to  temperature.  It  is  a  matter  of  common  obser- 
vation in  the  rearing  of  this  material  that  the  reproductive  age  is 
greatly  affected  by  temperature  and  food  conditions.  During  a  local 
period  of  high  temperature  and  warm  nights  in  summer,  a  generation 
may  be  obtained  in  6  to  7  days  in  S.  exspinosus  and  in  5  to  6  or  7 
days  in  Daphnia  and  even  in  1  or  2  days  in  Moina,  while  during  a 
period  of  continued  cool  days  and  nights  in  fall  or  spring,  when  the 
laboratory  is  not  provided  with  heat,  the  reproductive  age  stretches 
out  to  11  to  14  days  for  Simocephalus  and  Daphnia  and  8  to  12  days 
for  Moina. 

Curves  were  made  for  some  of  the  lines,  showing  the  mean  re- 
productive ages  for  different  two-month  periods.  At  times  they  show 
remarkably  close  agreement  in  mean  reproductive  ages,  not  only 
between  the  two  strains  of  the  same  line  but  also  between  the  different 
lines  of  the  same  species  and  to  a  considerable  degree  between  D. 
pulex  and  S.  exspinosus.  Figure  IB  (for  Line  695)  and  Figure  17s 
(for  Line  757)  show  two  of  these  curves. 

STATISTICAL  TREATMENT  OF  DATA. 

Irregularities  in  phototropic  response  are  common.  They  are 
noted  in  almost  every  paper  dealing  with  the  light-reactions  of  ani- 
mals. Dependable  results  are  generally  obtained  only  by  securing 
sufficient  data  to  obtain  averages  from  relatively  large  numbers. 

Like  most  other  material,  the  Cladocera  discussed  in  this  paper 
showed  considerable  irregularities  in  light-reactions,  and  compari- 
sons of  the  two  selected  strains  of  the  same  line  are  made  by  con- 
sidering the  data  by  longer  periods,  so  as  to  avoid  the  extreme  effect 
of  local  conditions  and  fluctuations  in  the  behavior  of  individual 
broods. 

In  the  study  of  a  character  so  complicated  as  a  behavior  charac- 
teristic it  is  essential  to  deal  with  relatively  large  numbers,  and  the 
safest  way  to  deal  with  large  series  of  data  is  by  statistical  treat- 
ment. Much  time  has  been  given  to  the  statistical  analysis  of  this 
data,  although  the  treatment  has  been  carried  only  so  far  as  seemed 


A   PHYSIOLOGICAL   CHARACTER.  27 

necessary  in  order  to  bring  out  the  real  significance  of  the  material,1 
and  has  been  confined  to  the  determination  of  standard  deviations, 
probable  errors,  and  a  few  correlations. 

Probable  errors  have  been  used  in  seeking  to  determine  the 
significance  of  the  reaction-time  differences  between  the  selected 
strains  of  the  various  lines.  Many  more  probable  errors  have  been 
determined  (and  included  in  the  tables  for,  the  benefit  of  the  reader) 
than  have  been  referred  to  in  the  text  discussions.  On  the  other 
hand,  probable  errors  have  not  been  determined  for  many  differences 
which  were  obviously  too  small  to  have  statistical  significance.2 

For  the  benefit  of  anyone  wishing  to  work  over  a  portion  of  the 
data  statistically,  the  sums  of  the  individual  reaction-times  and  the 
sums  of  the  squares  of  the  individual  reaction-times  are  given  in  the 
tables  which  present  the  data  for  Lines  695  and  757  by  broods. 
Hence,  most  of  the  statistical  treatment  for  these  two  lines  is  capable 
of  verification  from  the  data  presented  in  this  paper.  Likewise  the 
data  are  available  for  any  further  statistical  treatment. 

The  standard  deviations  of  the  reaction- time  means  are  large, 
whether  the  data  is  considered  by  shorter  periods  or  by  year  periods. 
This  is  largely  due  to  the  over- time  individuals,  the  data  for  which 
afforded  considerable  difficulty  in  statistical  treatment.  Other  plans 
of  handling  the  data  for  these  individuals  (other  than  assuming  900 
seconds  as  their  reaction-time)  were  considered,  but  they  seemed  to 
provide  no  better  method  of  utilizing  this  very  significant  part  of 
the  data. 

1  Such  statistical  treatment  as  has  been  accorded  the  data  of  this  paper  has  been  greatly 
facilitated  by  suggestions  from  Dr.  J.  Arthur  Harris,  of  the  Station  for  Experimental  Evolution. 
However,  any  errors  or  insufficient  treatment  must  be  ascribed  to  the  writer,  who  has  conducted 
the  statistical  analysis. 

2  It  is  recognized,  however,  that  the  value  of  the  probable  error  is  not  fully  utilized  when 
one  fails  to  determine  probable  errors  for  all  the  differences  in  a  series  of  data — for,  obviously, 
a  difference  which  is  twice    its    probable  error  deserves  more  consideration  than  one  which  is 
only  a  fraction  of  its  probable  error,  though  a  difference  less  than  2^  times  its  probable  error 
is  considered  of  doubtful  statistical  validity.    Nevertheless,  to  conserve  the  computer's  time,  it 
was  felt  advisable  to  omit  the  determination  of  probable  errors  for  many  of  the  differences 
which  did  not  promise  marked  statistical  value. 


28  SELECTION    IN    CLADOCERA    ON    THE    BASIS    OF 


ANALYSIS  OF  DATA. 

LINE  695. 
PRESENTATION  OF  DATA. 

The  data  for  Line  695  may  be  considered  first.  Line  695  is  one 
of  the  original  lines  taken  into  the  laboratory,  the  stock  having  been 
obtained  from  the  spring-fed  pond  in  the  woods  (Pond  II),  November 
17,  1911.  The  selection  was  begun  March  26,  1912.  The  stock  had 
then  descended  8  generations  in  the  laboratory.  By  that  time 
methods  of  rearing  and  handling  the  material  had  been  worked  out 
and  the  stock  was  well  established. 

The  manner  of  handling  the  young  in  making  the  selections  has 
already  been  discussed.  The  selection  data  complete,  summarized 
by  broods,  are  given  in  the  tables  for  the  695  plus  and  the  695  minus 
strains.  Tables  3  and  4  contain  in  successive  vertical  columns, 
beginning  at  the  left,  the  designation  of  the  generation;  the  date  of 
selection  from  within  the  brood;  the  hour  of  the  day  at  which  the 
experiment  was  conducted;  the  age  of  the  young,  whether  a  "new" 
brood  or  a  brood  released  a  day  earlier  (Sunday's  broods  and  rarely 
others  were  tested  when  a  day  old);  the  number  of  individuals  in 
the  brood;  the  temperature  of  the  water  in  the  experimental  tank  at 
the  time  of  the  experiment;  the  number  of  individuals  which  went 
to  the  negative  end  of  the  tank;  the  number  of  individuals  which 
failed  to  respond  to  the  extent  of  reaching  either  end  of  the  tank 
within  the  period  allotted  (15  minutes);  the  minimum  reaction-time 
for  an  individual  of  each  brood;  the  maximum  reaction-time;  the 
sum  of  the  individual  reaction-times  for  the  brood;  the  mean  reaction- 
time  for  the  brood;  and  finally,  the  sum  of  the  squares  of  the  indi- 
vidual reaction-times  (for  computation  purposes).1 

Consultation  of  tables  3  and  4  shows  that  there  are  wide  differ- 
ences in  reactiveness  of  the  various  broods.  These,  in  part  at  least, 
are  due  to  the  factors  discussed  on  pages  17-20  and  128-149  of  this 
paper.  It  is  unfortunate,  though  not  surprising,  that  these  reaction 
differences  are  so  large,  but  when  all  of  the  factors  of  the  case  are 
taken  into  account,  it  is  believed  that  they  are  not  unduly  disturbing 
to  the  course  of  the  experiment. 

Because  of  the  relatively  wide  differences  in  reaction-time  be- 
tween different  broods  and  the  small  numbers  in  many  of  the  broods, 

1  For  the  benefit  of  anyone  wishing  to  go  over  the  author's  analysis  or  make  further  analysis, 
it  was  desired  to  present  the  complete  data  for  all  the  lines  discussed  in  this  paper.    But  (in  view 
of  the  present  great  cost  of  printing  so  many  pages  of  tables)  data  are  presented  practically  com- 
plete for  Lines  695  and  757  only.    It  is  quite  impracticable  to  publish  the  individual  reaction- 
times  for  all  the  individuals  tested  in  making  the  selections,  even  for  Lines  695  and  757.    The  sums 
of  the  squares  of  these  reaction-times  are  given.    This  renders  available  all  the  data  necessary 
for  a  critical  analysis  of  the  original  data  and  should  satisfy  the  most  exacting  reader,  so  far  as 
Lines  695  and  757  are  concerned.    The  detailed  data  for  all  the  lines  have  been  typed  in  tabular 
m,  such  as  is  used  for  the  data  for  Lines  695  and  757,  and  are  stored  in  duplicate  at  the  Station 
Experimental  Evolution,  where  they  may  be  freely  consulted  by  anyone  interested. 


A   PHYSIOLOGICAL   CHARACTER. 


29 


TABLE  3. — Summary  of  selection  data  by  broods  for  Line  695  pli 


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| 

0 

.a 

•8 

1  . 

1 

1.9 

9 

e 

S 

!•§ 

i 

2 

£ 

1 

1 

1 

I*! 

g 

0  +i 

>>  a 

§ 

3 

o3 

}jj 

T3  ^ 

C  0 

a  «; 

g 

M 

1 

2 

"o  J 

*o 

•M 

L 

£  J 

•g.d 

li 

li 

•8"! 

£ 

"8  ro 

§ 

0> 

0 

I'J 

1 

4! 

o 
6 

a"« 

j] 

^ 

*1 

1  8 

§£ 

11 

s  « 

g  1 

CQ  °° 

1 

s  a 
I1 

1912 

H* 

Mar.  26 

3 

0 

8 

13.4°C 

0 

0 

90 

295 

1317 

177 

297039 

I 

April  6 

10 

2 

10 

14.5 

3 

2 

100 

900 

4850 

3108600 

J 

April  16 

12 

0 

9 

18.2 

0 

0 

130 

390 

2260 

^51 

615700 

Jo 

K 

April  23 
May  11 

4 
5 

0 
0  or  1 

5 
4 

16 
18.5 

0 
0 

1 

0 

360 
165 

900 
525 

3210 
1280 

642 
320 

2261700 
485350 

L 

M 

May  27 
June  3 

11 
1 

0  or  1 
0  or  1 

15 

7 

18.5 
21 

Sele 
1 

ction 
1 

s  ma< 
180 

lebu 
900 

;  record 
3445 

s  incom 
492 

plete. 
2080425 

N 

June  10 

11 

0  or  1 

7 

17 

0 

2 

285 

900 

4420 

631 

3200350 

O 

June  19 

3 

0 

2 

18.5 

0 

1 

590 

900 

1490 

745 

1158100 

P 

June  26 

10 

0 

2 

19.5 

0 

2 

900 

900 

1800 

900 

1620000 

8. 

July  3 
July  5 

3 
3 

0 
0 

1 
2 

19.5 
21.7 

0 
0 

0 
0 

260 
430 

260 
730 

260 
1160 

260 
580 

67600 
717800 

R 

July  15 

11 

0  or  1 

1 

22 

0 

0 

690 

690 

690 

690 

476100 

S 

July  30 

10 

0 

1 

20 

0 

0 

495 

495 

495 

495 

245025 

S« 

Aug.  2 

11 

0 

5 

19.5 

0 

0 

170 

570 

1880 

376 

841600 

S« 

Aug.  5 

.3 

0  or  1 

2 

18.7 

0 

0 

280 

310 

590 

295 

174500 

T 

Aug.  16 

2 

0 

4 

20.5 

0 

2 

95 

900 

2060 

515 

1656250 

U 

Aug.  28 

1 

0 

1 

19.5 

0 

0 

195 

195 

195 

195 

38025 

v° 

Sept.  9 

2 

Oor  1 

1 

20 

0 

0 

140 

140 

140 

140 

19600 

w 

Sept.  26 

2 

0 

4 

18 

0 

0 

70 

390 

795 

199 

214625 

X 

Oct.   7 

1 

0  or  1 

3 

15.7 

0 

0 

192 

296 

696 

232 

167744 

Y 

Oct.  18 

2 

0 

2 

18 

0 

0 

180 

315 

495 

£48 

131625 

Z 

Oct.  29 

1 

0 

3 

17 

0 

0 

88 

210 

458 

153 

77444 

A2 

Nov.  8 

2 

0 

2 

18 

0 

0 

165 

210 

375 

188 

71325 

B2 

Nov.  20 

1 

0 

2 

16.4 

0 

0 

244 

375 

619 

310 

200161 

C2 

Nov.  30 

1 

0 

2 

15.4 

0 

0 

135 

490 

625 

313 

258325 

D2 

Dec.  12 

1 

0 

8 

16 

0 

0 

90 

235 

1252 

157 

219504 

E2 

Dec.  20 

1 

0 

9 

18.8 

0 

0 

50 

270 

1315 

146 

222825 

F2 

Dec.  30 

11 

Oor  1 

16 

16 

0 

0 

80 

455 

3770 

236 

1169350 

1913 

G2 

Jan.   6 

11 

0  or  1 

8 

16.4 

0 

0 

80 

570 

2025 

253 

684375 

H2 

Jan.  20 

1 

0  or  1 

11 

15.8 

0 

0 

70 

625 

3259 

296 

1292531 

I2 

Jan.  27 

1 

0  or  1 

10 

15.5 

0 

0 

90 

415 

2140 

214 

566700 

Feb.  3 

2 

Oor  1 

7 

14.6 

0 

0 

150 

275 

1535 

205 

305675 

K22 

Feb.  12 

1 

0 

9 

13 

0 

0 

130 

315 

1895 

211 

432025 

L2 

Feb.  20 

1 

0 

10 

15.2 

0 

3 

160 

900 

5180 

518 

3410200 

Mi 

Feb.  28 

2 

0 

11 

16.2 

0 

0 

160 

530 

3165 

288 

1100025 

N2 

Mar.  10 

1 

0  or  1 

10 

15 

0 

0 

135 

425 

2600 

260 

764100 

O2 

Mar.  19 

2 

0 

3 

13.1 

0 

0 

110 

260 

500 

167 

96600 

Pj 

Mar.  28 

9 

2 

6 

13.3 

0 

0 

260 

425 

2155 

359 

797475 

Q* 

April  4 

2 

0 

7 

20.5 

0 

0 

110 

215 

1155 

165 

201275 

R2 

April  11 

2 

0 

12 

19 

1 

0 

100 

580 

3065 

255 

1080975 

82 

April  18 

4 

0 

10 

18.7 

0 

0 

60 

285 

1680 

168 

334250 

Ti 

April  25 

3 

0 

11 

18.4 

0 

0 

125 

450 

2725 

248 

781175 

U2 

May  5 

1 

0  or  1 

8 

17.5 

0 

0 

70 

190 

1010 

126 

141150 

V2 

May  12 

1 

0  or  1 

4 

14 

0 

0 

100 

230 

635 

159 

112225 

Wt 

May  20 

2 

0 

7 

14.5 

0 

0 

135 

450 

1695 

242 

477225 

X2 

May  28 

4 

0 

8 

16 

0 

0 

100 

245 

1380 

173 

254400 

Y2 

June  5 

1 

0 

2 

18 

0 

0 

110 

125 

235 

118 

27725 

Z2 

June  12 

3 

0 

8 

17.2 

0 

5 

225 

900 

5265 

658 

4248225 

As 

June  18 

1 

0 

7 

19.3 

2 

0 

130 

395 

1410 

201 

341350 

B3 

June  27 

10 

0 

5 

20.5 

0 

0 

80 

170 

630 

126 

85750 

C3 

July  3 

2 

0 

4 

22.3 

1 

0 

140 

870 

1750 

438 

1134350 

Ds 

July  10 

3 

2 

2 

22 

0 

0 

190 

400 

590 

295 

196100 

E3 

July  19 

2 

0 

2 

22.3 

0 

0 

520 

575 

1095 

548 

601025 

F3 

July  26 

11 

0 

1 

21.7 

0 

0 

370 

370 

370 

370 

136900 

Fj° 

July  29 

2 

0 

4 

22.5 

0 

0 

115 

356 

1010 

253 

285334 

G3 

Aug.  1 

2 

0 

6 

22.8 

0 

1 

165 

900 

3255 

543 

2167925 

H3 

Aug.  8 

3 

0 

13 

21.9 

0 

0 

90 

750 

4155 

319 

1699175 

13 

Aug.  14 

1 

0 

4 

0 

0 

100 

222 

564 

141 

88638 

J3 

Aug.  20 

2 

0 

7 

20  '.4'" 

0 

0 

153 

543 

2215 

316 

847305 

Kj 

Aug.  26 

3 

0 

10 

20 

1 

0 

75 

497 

2037 

204 

565827 

L3 

Sept.  3 

10 

1 

4 

22.5 

1 

0 

80 

215 

603 

151 

100299 

•This  is  the  original  brood  from  which  selections  were  made  for  the  beginning  of  both  the  plus  and 
minus  strains. 

b  Mean  reaction-times  in  italics  are  those  for  which  there  is  a  corresponding  mean  for  a  brood  tested  on 
the  same  day  in  the  minus  strain. 

0  Selection  was  repeated  from  the  later  brood  because  of  the  loss  of  the  earlier  brood. 


30 


SELECTION    IN    CLADOCERA    ON   THE    BASIS   OF 


TABLE  3. — Summary  of  selection  data  by  broods  for  Line  695  plus — Continued. 


1 

1 

3 

A 

o 

a 

.2 

.2 

4 

& 

2 

g 

to 

sf 

o 

o 

• 

§ 

-.2 

£ 

0 

J 

Q 

,a 

e 

g 

I 

2 

| 

1 

1 

fl) 

H 

'3  ^ 

i 

'-S 

K 

a 

H 

O 

c 

1 

0 

0. 

m 

0> 

X 

i£ 

| 

1 

1 

j 

•o 

1 

1$ 

G 

.9 

f 

3*8 

1 

i 

.2 

J 

1 

| 

o 

fri 

co  O 

o. 

H 

V 

|.s 

I 

2 

1* 

11 

a  « 

!« 

^  «6 

1 

1 

i 

11 

*0 

§ 

•s| 

0 

6 

h 

i 

:§  1 

axiin 
econd 

II 

a 
3 

s  a 

0 

Q*1 

w 

*ft 

i 

H~ 

£^ 

|g 

£  • 

S" 

£S 

S 

3  '-£ 
OQ 

1913 

Ml 

Sept.  9 

2 

0 

9 

20.6°C. 

0 

0 

130 

549 

2475 

£76 

892071 

Nl 

Sept.  17 

10 

0 

8 

18.4 

0 

2 

130 

900 

2759 

S45 

1783651 

0, 

Sept.  27 

10 

0 

13 

16.8 

0 

0 

106 

352 

2674 

206 

614380 

Pi 

Oct.   3 

11 

0 

5 

17.9 

1 

0 

156 

455 

1283 

257 

390103 

Qi 

Oct.   9 

10 

0 

13 

20.1 

1 

0 

110 

576 

2813 

216 

790577 

RJ 

Oct.  16 

9 

0 

9 

.17.5 

0 

0 

173 

405 

2402 

267 

695916 

Si 

Oct.  23 

11 

0 

7 

15.3 

0 

0 

91 

340 

1184 

169 

242080 

Ti 

Oct.  30 

11 

0 

6 

16.7 

0 

0 

113 

435 

1208 

201 

314160 

U, 

Nov.  13 

11 

0 

5 

15.1 

0 

0 

91 

318 

778 

156 

158166 

Vi 

Nov.  20 

2 

0 

12 

18.8 

0 

0 

93 

365 

2276 

190 

505048 

W3 

Nov.  27 

11 

0 

9 

12.7 

0 

1 

234 

900 

4075 

453 

2294323 

X, 

Dec.  5 

11 

0 

5 

13.8 

0 

0 

358 

691 

2447 

489 

1281475 

Y, 

Dec.  13 

10 

0 

4 

12.5 

0 

2 

308 

900 

2618 

655 

1974964 

Zi 

Dec.  22 

11 

1 

8 

12.3 

0 

1 

320 

900 

4211 

526 

2450339 

A4 

Dec.  27 

6 

Se 

ectio 

is  mi 

deb 

it  record  incomnletfi. 

1914 

B« 

Jan.   5 

7 

1 

6 

11.9 

0 

0 

150 

560 

1840 

307 

669850 

C4 

Jan.  15 

2 

0 

8 

13 

J 

1 

340 

900 

4200 

525 

2422408 

D« 

Jan.  22 

1 

0 

7 

13.8 

0 

1 

158 

900 

3486 

498 

2103652 

Bt 

Jan.  30 

1 

0 

12 

15.8 

3 

2 

135 

900 

6451 

538 

4267309 

F« 

Feb.  5 

2 

0 

7 

14 

1 

0 

183 

617 

2519 

360 

1025103 

(U 

Feb.  12 

3 

0 

6 

12.8 

2 

1 

299 

900 

3347 

558 

2069435 

Hi 

Feb.  20 

5 

0 

18 

16.2 

0 

3 

110 

900 

9963 

554 

6732879 

Mar.  2 

4 

1 

5 

16.3 

0 

0 

85 

238 

825 

165 

149961 

J*4 

Mar.  9 

5 

0 

12 

18.3 

1 

0 

53 

250 

1889 

157 

336795 

K4 

Mar.  21 

3 

0 

8 

17 

0 

6 

219 

900 

5900 

738 

4986922 

April  2 

2 

0 

5 

18.2 

0 

0 

105 

570 

1345 

269 

495625 

M4 

April  9 

2 

0 

9 

18.5 

0 

2 

120 

900 

3900 

433 

2334050 

N4 

April  16 

3 

0 

10 

19 

0 

2 

85 

900 

3195 

320 

1906975 

04 

April  24 

1 

0 

6 

17 

0 

0 

120 

630 

1455 

243 

539625 

P4 

May  1 

2 

0 

12 

19 

4 

2 

183 

900 

6317 

526 

4182749 

Q« 

May  7 

6 

0 

15 

20.8 

11 

1 

95 

900 

5536 

369 

3003036 

Hi 

May  14 

1 

0 

1 

21.2 

0 

0 

297 

297 

297 

£97 

88209 

Bi 

May  20 

4 

0 

9 

21.2 

3 

j 

135 

900 

3728 

414 

2125440 

T< 

May  27 

4 

0 

8 

19.9 

0 

7 

645 

900 

6945 

868 

6086025 

U4 

June  3 

2 

0 

5 

18.6 

0 

220 

850 

2487 

497 

1484789 

June  27 

1 

0 

3 

21 

1 

1 

130 

900 

1335 

445 

919925 

W4 

July  3 

1 

0 

4 

19 

0 

0 

160 

345 

985 

246 

259825 

X4 

July  10 

1 

0 

6 

21 

0 

0 

170 

735 

3230 

538 

1925400 

Y4 

July  16 

2 

0 

10 

23 

0 

0 

120 

520 

3020 

302 

1111700 

July  22 

5 

0 

7 

21.1 

0 

0 

75 

604 

2091 

299 

864455 

At 

July  28 

11 

0 

5 

21 

4 

J 

165 

900 

1885 

377 

1101575 

Bs 

Aug.  5 

2 

0 

6 

22 

0 

4 

285 

900 

4355 

726 

3542125 

Ci 

Aug.  11 

2 

0 

2 

24.8 

0 

1 

180 

900 

1080 

540 

842400 

Aug.  20 

1 

0 

5 

23 

1 

0 

201 

525 

1633 

327 

592355 

Ei 

Aug.  26 

7 

0 

8 

21 

0 

2 

125 

900 

3907 

488 

2562427 

Fi 

Sept.  1 

2 

0 

12 

22 

2 

2 

180 

900 

5780 

482 

3464606 

Gi 

Sept.  7 

4 

0 

9 

22 

1 

0 

90 

700 

2737 

304 

1213419 

Hi 

Sept.  16 

3 

0 

33 

16.2 

0 

8 

59 

900 

16589 

503 

11051527 

h 

Sept.  23 

3 

0 

8 

18.9 

1 

0 

70 

425 

1398 

176 

389734 

Ji 

Sept.  30 

2 

0 

14 

20.1 

3 

1 

178 

900 

5921 

42S 

3114431 

K, 

Oct.   8 

3 

0 

6 

21.8 

2 

1 

348 

900 

4046 

674 

2912988 

U 

Oct.  15 

1 

0 

15 

20.1 

2 

0 

70 

745 

4523 

302 

1951685 

Mi 

Oct.  21 

11 

0 

13 

18.8 

2 

0 

68 

480 

3054 

235 

917626 

Ni 

Oct.  28 

4 

0 

15 

17.5 

4 

2 

125 

900 

6892 

460 

4112772 

Oi 

Nov.  4 

6 

0 

14 

18.2 

2 

1 

82 

900 

3927 

281 

1759315 

P. 

Nov.  11 

4 

0 

14 

19.2 

0 

0 

100 

570 

3050 

218 

904678 

Qi 

Nov.  18 

4 

0 

15 

17.3 

6 

0 

115 

875 

4791 

319 

2206373 

K 

Nov.  26 

2 

0 

14 

18.5 

9 

0 

100 

840 

5429 

388 

3134113 

St 

Dec.  2 

4 

0 

5 

18.7 

0 

0 

80 

275 

875 

175 

180975 

Tl 

Dec.  8 

3 

0 

8 

16.7 

1 

0 

56 

263 

1136 

142 

209724 

Ui 

Dec.  14 

4 

0 

6 

21 

6 

0 

165 

520 

1943 

324 

740549 

lire.  21 

3 

0 

10 

15.7 

0 

0 

103 

235 

1388 

139 

205034 

w, 

Dec.  29 

3 

0 

10 

18.7 

0 

2 

95 

900 

4178 

418 

2592880 

1915 

X5 

Jan.   6 

1 

0 

9 

19 

0 

1 

90 

900 

2345 

261 

1088025 

Yi 

Jan.  15 

1 

0  . 

6 

18.9 

0 

1 

125 

900 

2725 

454 

1764225 

Jan.  23 

1 

0 

12 

18.9 

1 

3 

176 

900 

6136 

511 

4070576 

A    PHYSIOLOGICAL   CHARACTER. 


31 


TABLE  3.— Summary  of  selection  data  by  broods  for  Line  695  plus— Continued. 


a 
1 

L 

0> 

1 

a 

3 

.3 

.s 

.3 

1 

A 

P 

•8 

a 

a 

3 

«} 

e 

o 

a 

a 

i 

^5 

m 

a 

a 

i 

i 

w 

'3,  ^ 

a 

•43 

_g 

t 

8 

1 

o 
a 

1 

a 

I 

to  .J 

1 

i 

•§ 

i 
1 

'» 

a 

"8 

| 

1 

1 

& 
g 
-3 

of  young 
iment  in  d 

i 

1 

*o 

perature  i: 
k. 

of  negativ* 
iduals. 

ji 

S  c 

„ 
If 

I 

j. 

T 

1 

g 

"08  Js 

I 

g  ! 

6 

a  c 

6  •- 

O  c 

.3  8 

li 

si 

§ 

a  § 

O 

Q" 

N 

%  a 

K 

h 

fc* 

SS 

s  " 

3   «Q 

OJ 

a 

eg* 

1915 

Ae 

Feb.   1 

3 

0 

8 

17.7°C. 

0 

0 

192 

46. 

2343 

293 

737297 

Be 

Feb.  10 

1 

0 

9 

13.3 

0 

0 

77 

58C 

1648 

183 

492636 

Ce 

Feb.  18 

11 

0 

15 

16.5 

1 

i 

140 

90C 

5746 

383 

2826876 

De 

Feb.  26 

1 

0 

12 

19.2 

0 

0 

58 

267 

1818 

152 

334294 

Ee 

Mar.  6 

6 

0 

10 

19.3 

3 

0 

118 

633 

3393 

339 

1397655 

Fe 

Mar.  13 

2 

0 

18 

17 

0 

1 

70 

900 

3387 

188 

1262947 

Ge 

Mar.  22 

4 

0 

13 

17.6 

1 

2 

140 

900 

7787 

599 

5350389 

He 

April  1 

3 

0 

8 

20.8 

1 

1 

72 

900 

2650 

331 

1381078 

Ie 

April  8 

9 

0 

10 

18.5 

5 

2 

417 

900 

6668 

667 

4665470 

J. 

April  15 

3 

0 

15 

17.9 

0 

0 

60 

845 

3485 

232 

1481175 

Ke 

April  23 

2 

0 

14 

18 

0 

3 

83 

900 

6573 

470 

4300045 

Lt 

April  30 

2 

0 

10 

20.4 

1 

3 

125 

900 

4271 

427 

2824692 

Me 

May  8 

5 

0 

9 

19.7 

0 

1 

320 

900 

5634 

626 

3895706 

Ne 

May  17 

3 

0 

10 

16.9 

5 

0 

160 

760 

3300 

330 

1441050 

o» 

May  26 

3 

0 

5 

17 

1 

1 

205 

900 

2160 

432 

1251150 

P« 

June  5 

11 

0 

10 

16.8 

0 

0 

'108 

245 

1523 

152 

248689 

Q, 

June  12 

3 

0 

13 

19.4 

1 

1 

82 

900 

3752 

289 

1877124 

Re 

June  19 

2 

0 

12 

21.3 

1 

2 

140 

900 

5340 

445 

3  195400 

Se 

June  28 

3 

0 

4 

18 

1 

2 

245 

900 

2545 

636 

1930025 

Te 

July  5 

3 

0 

4 

21 

1 

0 

505 

670 

2401 

600 

1457641 

Ue 

July  13 

Random  distribution. 

Ve 

July  18 

Random  distribution. 

W« 

July  27 

Random  distribution. 

Xfi* 

Random  distribution. 

Ye 

Aug.  16 

5     1      13 

23.5 

8 

3 

90 

900 

5785 

445 

3766975 

Ze 

Aug.  26 

Random  distribution. 

AT 

Sept.  6 

5 

0 

2 

20.8 

0 

0 

180 

335 

515 

258 

144625 

BT 

Sept.  14 

5 

0 

5 

21.9 

0 

0 

120 

450 

1160 

232 

336200 

CT 

Sept.  20 

4 

0 

6 

20.3 

0 

0 

160 

580 

2070 

390 

823300 

D7 

Oct.   1 

4 

0 

9 

18.8 

0 

0 

100 

560 

2150 

239 

713350 

E7 

Oct.   9 

11 

0 

8 

17 

0 

1 

110 

900 

4080 

610 

2914000 

FT 

Oct.  20 

5 

0 

12 

18.1 

0 

0 

70 

855 

2760 

230 

1322900 

G7 

Oct.  28 

4 

0 

14 

15.5 

0 

0 

50 

260 

2030 

145 

347900 

H7 

Nov.  5 

5 

0 

18 

15 

3 

0 

70 

710 

4610 

256 

1623150 

IT 

Nov.  15 

4 

1 

18 

15.9 

1 

1 

60 

900 

4005 

223 

1855475 

JT 

Nov.  24 

6 

0 

28 

15.1 

1 

0 

60 

450 

5665 

202 

1566775 

KT 

Dec.  2 

4 

0 

17 

14.5 

0 

0 

50 

760 

4140 

244 

1597800 

L7 

Dec.  9 

5 

0 

12 

15.2 

1 

0 

90 

390 

2160 

180 

508800 

M7 

Dec.  16 

5 

0 

9 

18.1 

1 

1 

60 

900 

1935 

215 

954725 

NT 

Dec.  23 

4 

0 

15 

12.8 

2 

0 

100 

470 

3720 

248 

1068550 

1916 

O7 

Jan.   1 

Random  distribution. 

• 

PT 

Jan.  13 

8 

0 

7 

16 

0 

2 

100 

900 

2875 

411 

1890875 

Q7 

Jan.  23 

8 

1 

7 

18.2 

4 

0 

75 

180 

820 

117 

103150 

RT 

Jan.  29 

5 

0 

13 

12 

1 

2 

110 

900 

4060 

312 

2127250 

ST 

Feb.  7 

6 

0 

15 

14.7 

2 

0 

60 

635 

3605 

240 

1236625 

T7 

Feb.  16 

5 

0 

20 

17 

3 

2 

90 

900 

6205 

310 

2922075 

U7 

Feb.  23 

3 

0 

9 

11.4 

0 

0 

100 

570 

2095 

233 

667125 

V7 

Mar.  2 

4 

0 

14 

14.5 

0 

1 

90 

900 

2795 

200 

1122575 

W7 

Mar.  9 

5 

0 

9 

17.3 

0 

2 

145 

900 

3560 

396 

2175200 

X7 

Mar.  18 

7 

1 

12 

Sel 

ctions  made,  but  light  current  interrupted 

before  experiment  was  completed 

Y7 

Mar.  27 

3 

0 

3 

14.8 

0 

0 

60 

80 

210 

70 

14900 

ZT 

April  8 

3 

0 

13 

14.1 

5 

0 

95 

690 

3990 

307 

1701000 

Ag 

April  19 

3 

0 

17 

15 

0 

4 

90 

900 

6265 

369 

3947425 

Bg 

May  1 

4 

1 

6 

15.7 

1 

0 

80 

430 

1120 

187 

288000 

C8 

May  9 

4 

0 

13 

16.9 

0 

2 

110 

900 

4560 

361 

2479600 

Dg 

May  18 

3 

0 

8 

17.9 

0 

2 

60 

900 

2620 

328 

1763400 

Eg 

May  26 

3 

0 

10 

18.8 

0 

2 

90 

900 

3535 

354 

2141575 

Fg 

June  3 

5 

0 

5 

19.1 

1 

0 

135 

260 

1065 

213 

235925 

Gg 

June  12 

4 

0 

7 

17.3 

0 

0 

145 

420 

2185 

312 

735775 

Hg 

June  20 

Random  distribution. 

Ig 

July  8 

3 

1 

8 

20 

3 

4 

270 

900 

4810 

601 

3607700 

Jg 

July  12 

5 

1 

3 

22.7 

0 

0 

180 

290 

720 

240 

179000 

Kg 

July  18 

4 

0 

20 

21.3 

2 

7 

150 

900 

10600 

530 

7431800 

Lg 

July  24 

3 

0 

6 

21.4 

0 

1 

100 

900 

1820 

303 

1053000 

Mg 

Aug.  1 

2 

1 

14 

20.9 

0 

9 

250 

900 

9810 

701 

7912700 

Ng 

Aug.  7 

2 

0 

8 

23.2 

0 

7 

300 

900 

6600 

825 

5760000 

Og 

Aug.  14 

4 

1 

14 

18.7 

2 

1 

90 

900 

3670 

262 

1816506 

Pg 

Aug.  19 

2 

0 

8 

20.3 

0 

4 

100 

900 

4110 

614 

3308900 

Qi 

Aug.  26 

3 

0 

33 

21 

0 

7 

140 

900 

13320 

404 

8776400 

Rg 

Sept.  2 

2 

0 

12 

20.2 

4 

2 

160 

900 

4920 

410 

2854800 

•All  of  the  695+  strain  was  lost  except  a  substrain  of  695+  which  had  been  kept  in  a  dark  closet  for  13 
generations.    Renewed  from  695  Xs+  dark. 


32  SELECTION   IN   CLADOCERA    ON   THE    BASIS   OF 

TABLE  4. — Summary  of  selection  data  by  broods  for  Line  695  minus. 


1 
1 

*o 

a 
1 

ing  in- 

o 

i 

I 

a 

.9 
8 

a 

1 

A 

Q 

a 

•d 

0 

Is  •* 

a 

'•S 

a 

2 

a 

"3 
.1 

• 

a 

a> 
C3 

1 

*i 

a 

a 
.2 
i 

J 

I 

" 

••g 

Wa 

> 

II 

1 

1 

"o 

| 

£3 

1 

i| 

a 

>>"S 

3 
0 

1 

11 

15 

a  „ 

a 

s  ^ 

1 

"o  J 

*o 

«*H 

"8 

«  • 

o  IH 

11 

11 

13  "H 

a 

"3  « 

1 

Js 

w 

|I 

6 

|| 

H  ** 

&* 

6  1 

11 

|i 

JS 

1 

a.  a 

1912 

I 

April  6 

10 

2 

10 

14.5°C 

2 

2 

200 

900 

5385 

639 

3491975 

J 

April  17 

11 

0 

3 

16.6 

0 

1 

165 

900 

1537 

512 

1060009 

K 

May  6 

11 

Oor  1 

6 

16.3 

0 

4 

215 

900 

4250 

708 

3475450 

L 

May  15 

5 

0 

6 

0 

5 

590 

900 

5090 

848 

4398100 

L* 

May  20 

3 

0 

1 

17 

0 

0 

230 

230 

230 

230 

52900 

M 

May  25 

4 

0 

5 

19 

0 

0 

185 

645 

1485 

297 

594875 

N 

June  1 

10 

0 

12 

18.5 

0 

1 

80 

900 

2815 

235 

1243425 

O 

June  10 

11 

Oorl 

5 

17 

0 

3 

520 

900 

4110 

822 

3492500 

P 

June  18 

1 

0 

4 

19 

0 

0 

180 

330 

935 

234 

231625 

Q 

July  1 

11 

Oor  1 

3 

18 

0 

0 

320 

790 

1495 

498 

874725 

R 

July  8 

10 

0  or  1 

2 

22 

0 

1 

290 

900 

1190 

595 

894100 

R* 

July  9 

11 

0 

2 

22 

0 

0 

230 

345 

595 

288 

171925 

S 

July  17 

9 

0 

2 

22 

0 

1 

450 

900 

1350 

675 

1012500 

H" 

Aug.  1 

3 

0 

4 

21 

2 

600 

900 

3040 

760 

2389600 

T 

Aug.  16 

2 

0 

1 

20.5 

0 

0 

235 

235 

235 

235 

55225 

U 

Sept.  9 

2 

Oorl 

1 

20 

0 

0 

285 

285 

285 

285 

81225 

V 

Sept.  16 

2 

0 

1 

19 

0 

0 

255 

255 

255 

255 

65025 

V* 

Sept.  20 

5 

0 

1 

19.8 

0 

0 

460 

460 

460 

460 

211600 

w 

Oct.  2 

1 

0 

6 

14.1 

2 

0 

95 

665 

1868 

311 

796624 

X 

Oct.  15 

2 

0 

1 

19.2 

0 

0 

280 

280 

280 

280 

78400 

x« 

Oct.  18 

3 

0 

8 

18 

1 

0 

70 

615 

2657 

332 

1150719 

Y 

Nov.  1 

1 

0 

1 

18 

0 

0 

174 

174 

174 

174 

30276 

z 

Nov.  16 

1 

0 

2 

15 

0 

0 

235 

445 

680 

340 

253250 

At 

Nov.  25 

2 

Oorl 

4 

17 

0 

0 

310 

595 

1945 

486 

991725 

Bt 

Dec.  7 

3 

0 

5 

16 

0 

0 

170 

230 

980 

196 

194350 

Ci 

Dec.  17 

2 

0 

12 

19 

0 

0 

105 

605 

4372 

364 

1987924 

D» 

Dec.  26 

1 

0 

11 

14 

0 

0 

175 

500 

3058 

278 

790954 

1913 

EJ 

Jan.  6 

11 

Oor  1 

14 

16.4 

0 

2 

75 

900 

4967 

361 

2819819 

F, 

Jan.  13 

1 

Oor  1 

4 

14.4 

0 

0 

90 

190 

555 

139 

84825 

Gs 

Jan.  21 

2 

0 

8 

14.4 

0 

0 

125 

280 

1625 

203 

356475 

Ht 

Jan.  28 

3 

0 

8 

13.4 

0 

0 

95 

365 

1935 

242 

540675 

It 

Feb.  6 

2 

0 

9 

13 

0 

0 

100 

355 

2075 

231 

553475 

J, 

Feb.  17 

1 

Oor  1 

9 

13.4 

0 

0 

140 

510 

2970 

330 

1066175 

Kz 

Feb.  24 

11 

Oor  1 

7 

12.5 

0 

0 

250 

450 

2470 

353 

908750 

L, 

Mar.  3 

11 

Oor  1 

8 

11.8 

1 

0 

110 

340 

1675 

209 

416125 

Ms 

Mar.  20 

1 

0 

1 

17 

0 

0 

315 

315 

315 

315 

99225 

N, 

April  7 

4 

Oor  1 

4 

14.8 

0 

0 

100 

200 

610 

153 

98850 

Ot 

April  16 

3 

0 

11 

17.5 

0 

0 

110 

505 

2600 

236 

777078 

Pt 

April  24 

2 

0 

14 

17.5 

3 

0 

80 

395 

3145 

225 

814325 

Qi 

May  2 

3 

Oorl 

6 

15.8 

0 

0 

95 

250 

975 

163 

173975 

R» 

May  12 

1 

Oor  1 

7 

14 

0 

0 

210 

460 

2140 

306 

699950 

81 

May  20 

2 

0 

5 

14.5 

0 

0 

330 

660 

2155 

431 

1002625 

T, 

May  29 

3 

0 

6 

16.5 

0 

0 

70 

135 

640 

107 

71850 

Ut 

June  5 

1 

0 

6 

18 

0 

0 

130 

250 

1105 

184 

212525 

V, 

June  12 

4 

0 

9 

17.2 

0 

1 

95 

900 

2795 

311 

1364175 

W, 

June  18 

2 

0 

5 

19.3 

0 

0 

130 

610 

1625 

325 

702425 

X, 

June  24 

2 

0 

6 

20.5 

1 

1 

207 

900 

3067 

511 

1971599 

Y» 

July  1 

2 

0 

1 

21.7 

0 

0 

138 

138 

138 

138 

19044 

Zt 

July  10 

3 

0 

2 

22 

0 

0 

290 

800 

1090 

645 

724100 

Aa 

July  19 

2 

0 

4 

22.3 

1 

1 

125 

900 

1635 

409 

1011675 

Ba 

July  25 

5 

0 

5 

21.8 

0 

1 

165 

900 

1945 

389 

1101375 

C, 

2 

0 

5 

22.8 

0 

2 

430 

900 

3280 

656 

2356950 

Da 

Aug.  8 

4 

0 

10 

21.9 

0 

0 

150 

345 

2180 

218 

516350 

Ea 

Aug.  14 

2 

0 

11 

0 

0 

80 

635 

2630 

239 

889150 

F, 

Aug.  20 

3 

0 

2 

20.4'" 

0 

1 

850 

900 

1750 

875 

1532500 

G, 

AUK.  28 

4 

0  or  1 

11 

21 

0 

0 

100 

320 

2083 

189 

438627 

Ha 

Kept.  3 

10 

1 

9 

22.5 

4 

0 

67 

458 

1466 

163 

360146 

la 

Sept.  9 

2 

0 

13 

20.6 

0 

1 

122 

900 

5102 

393 

2701034 

J> 

Sept.  17 

10 

0 

8 

18.4 

0 

0 

146 

460 

1982 

248 

564172 

K, 

Sept.  22 

11 

0 

10 

20 

o- 

1 

113 

900 

3011 

301 

1414841 

Sept.  29 

10 

1 

6 

16.5 

0 

0 

113 

640 

1595 

266 

629195 

M, 

Ort.  13 

4 

0 

9 

16.6 

1 

0 

123 

620 

2565 

285 

961813 

Na 

Oct.  20 

11 

0 

6 

17.8 

0 

0 

110 

244 

1092 

182 

212072 

0, 

Oct.  28 

11 

0 

8 

19.3 

0 

0 

94 

370 

1284 

161 

261488 

Pa 

Nov.  14 

11 

0 

7 

16 

3 

0 

74 

440 

1411 

202 

363415 

Q. 

Nov.  20 

11 

0 

7 

16.9 

0 

0 

122 

510 

2069 

296 

726711 

•Selection  was  repeated  from  a  later  brood  because  of  the  loss  of  the  earlier  brood. 


A   PHYSIOLOGICAL   CHARACTER.  33 

TABLE  4. — Summary  of  selection  data  by  broods  for  Line  695  minus — Continued. 


1 

A 

B 

3 

A 

o 

a 

.a 

.s 

i 

a 

1 

o 

V 

1 

S 

M 

a 

o 

a 

1 

1 

1 

1 

2 

I 

i 

"o 

1-2 

•Jj 

« 

1 

a 
| 

o 
a 

! 

a 

Jb 

to 

•3* 

3  O 

J 

1 

§ 

1 

'•3 

•3 

1 

1  . 
1| 

1 

3  . 

*>  9 

g 

§ 

E 

03 

i 

I  c 

a  « 

I 
I. 

| 

1 

1 

"oJB 

"o 

ft> 

"s  a 

^J 

"8 

s* 

•"  T 

0 

!"§ 

3  « 
Ji 

"*1j 

I 

°  3 

1 

.g  w 

a* 

W 

jl 

d 

P 

1- 

c 
d  a 

55  * 

|! 

P 

Ii 

1 

CO  ** 

1913 

Rj 

Dec.  1 

Random  distribution. 

Dec.  8 

11 

0 

5 

15.5°C 

0 

.  0 

188 

380 

1468 

294 

454376 

T» 

Dec.  16 

1 

0 

5 

17.5 

0 

0 

210 

430 

1615 

323 

553075 

Us 

Dec.  26 
1914 

5 

0 

8 

11.8 

0 

1 

310 

900 

3756 

470 

2012560 

V, 

Jan.   4 

10 

1 

8 

12 

0 

2 

280 

900 

4120 

515 

2628850 

Wj 

Jan.  12 

1 

0 

8 

11.8 

0 

0 

223 

665 

3220 

403 

1452092 

Xi 

Jan.  20 

4 

0 

8 

14.9 

0 

1 

205 

900 

3894 

487 

2379668 

Ys 

Jan.  28 

2 

0 

3 

13.4 

0 

1 

339 

900 

1609 

536 

1061821 

Zi 

Feb.  3 

3 

0 

11 

15.2 

1 

2 

230 

900 

6082 

553 

4014612 

A4 

Feb.  11 

2 

0 

8 

14.7 

0 

2 

210 

900 

4843 

605 

3443159 

B4 

Feb.  19 

1 

0 

14 

14.9 

1 

0 

102 

740 

4167 

298 

1651857 

C4 

Feb.  28 

4 

0 

5 

16.5 

0 

0 

153 

363 

1405 

281 

426847 

D4 

Mar.  7 

2 

0 

8 

17.5 

0 

1 

152 

900 

2963 

370 

1602127 

E4 

Mar.  16 

3 

0 

2 

17.3 

0 

0 

152 

221 

373 

187 

71945 

F4 

Mar.  24 

3 

0 

8 

16 

2 

2 

128 

900 

3512 

439 

2160278 

G« 

Mar.  31 

3 

0 

4 

16.3 

0 

2 

430 

900 

3020 

755 

2429000 

H4 

April  8 

1 

0 

4 

18.6 

0 

0 

145 

340 

915 

229 

233125 

J*4 

April  15 
May  2 

5 
3 

0 
0 

8 

1 

19.7 
19.7 

0 

1 

0 
0 

120 
490 

820 
490 

3340 
490 

418 
490 

1937200 
240100 

K4 

May  8 

4 

0 

11 

19 

6 

1 

88 

900 

3603 

328 

1704519 

L4 

May  14 

2 

0 

8 

21.2 

0 

0 

100 

360 

1656 

207 

410010 

M4 

May  22 

2 

0 

9 

18.7 

4 

I 

97 

900 

4224 

469 

2663016 

N4 

May  29 

11 

1 

7 

22 

1 

4 

490 

900 

5608 

801 

4634574 

04 

June  6 

2 

0 

2 

17.9 

0 

0 

535 

610 

1145 

573 

658325 

P4 

June  23 

11 

0 

9 

19.4 

3 

3 

65 

900 

4325 

481 

2957225 

i 

June  29 
July  8 

1 

2 

1 

0 

3 

7 

19.3 
19.6 

2 
1 

1 
1 

555 
305 

900 
900 

2051 
3550 

684 
507 

1473241 
2022850 

s* 

July  14 

2 

0 

6 

21.6 

0 

0 

115 

340 

1366 

228 

341958 

T4 

July  23 

2 

0 

1 

21.2 

1 

0 

141 

141 

141 

141 

19881 

U« 

July  30 

1 

0 

1 

19.2 

0 

1 

900 

900 

900 

900 

810000 

V4 

Aug.  8 

3 

0 

9 

22.1 

0 

2 

190 

900 

5016 

557 

3356106 

w« 

Aug.  15 

2 

0 

22 

22 

6 

5 

100 

900 

0052 

457 

6548080 

X4 

Aug.  22 

5 

0 

2 

22.9 

0 

1 

630 

900 

1530 

765 

1206900 

Y4 

Aug.  31 

2 

0 

8 

21.1 

4 

2 

123 

900 

3123 

390 

1939427 

Z4 

Sept.  7 

5 

0 

16 

21.8 

2 

0 

100 

800 

6383 

399 

3386309 

A5 

Sept.  17 

11 

0 

6 

16.3 

0 

0 

85 

395 

1111 

185 

266303 

B» 

Sept.  23 

3 

0 

11 

19 

8 

1 

100 

900 

3372 

307 

1670024 

c» 

Sept.  30 

2 

0 

8 

20.1 

2 

0 

80 

400 

1701 

218 

468945 

D, 

Oct.   7 

5 

0 

6 

20.1 

3 

0 

60 

370 

1020 

170 

247950 

E, 

Oct.  13 

3 

0 

15 

19 

2 

1 

74 

900 

6135 

409 

3623985 

Fi 

Oct.  20 

2 

0 

14 

19.9 

0 

2 

131 

900 

5108 

365 

2750448 

G. 

Oct.  26 

11 

0 

13 

17.3 

0 

0 

90 

550 

3285 

253 

1063923 

Hi 

Nov.  2 

3 

0 

12 

16.2 

4 

0 

80 

590 

3020 

252 

1119750 

Ii 

Nov.  9 

5 

0 

7 

16.7 

2 

0 

150 

620 

2125 

304 

886375 

Ji 

Nov.  16 

4 

1 

14 

19 

5 

0 

70 

670 

3123 

223 

1104069 

K, 

Nov.  23 

3 

0 

8 

17.3 

4 

0 

135 

680 

2005 

251 

722733 

L, 

Nov.  30 

5 

1 

34 

19 

8 

0 

60 

552 

8346 

246 

2771592 

Mj 

Dec.  5 

1 

0 

5 

18.4 

0 

0 

60 

188 

613 

123 

85569 

N. 

Dec.  11 

4 

0 

5 

16 

1 

2 

145 

900 

2435 

487 

1766693 

0, 

Dec.  19 

2 

0 

12 

16 

4 

1 

100 

900 

4350 

363 

2196550 

P. 

Dec.  26 

2 

0 

11 

12.5 

0 

0 

72 

280 

2307 

210 

•  522109 

1915 

Q« 

Jan.  4 

3 

1 

20 

19.3 

1 

0 

91 

735 

5364 

268 

1914546 

R, 

Jan.  11 

3 

0 

14 

20.7 

3 

1 

205 

900 

6180 

441 

3273950 

85 

Jan.  19 

3 

0 

7 

17.2 

0 

0 

65 

330 

1140 

163 

252150 

T8 

Feb.   1 

3 

1 

11 

17.5 

3 

0 

100 

630 

3439 

313 

1296451 

u. 

Feb.  10 

1 

0 

10 

13.3 

0 

0 

85 

445 

2340 

234 

678550 

V. 

Feb.  18 

10 

0 

12 

16.6 

2 

2 

110 

900 

3977 

331 

2184369 

ws 

Feb.  24 

3 

0 

10 

20.2 

1 

0 

105 

338 

1942 

194 

439790 

X. 

Mar.  5 

3 

0 

13 

16.3 

0 

0 

55 

395 

2415 

186 

621425 

Yt 

Mar.  12 

1 

0 

12 

16 

0 

1 

70 

900 

3052 

254 

1429104 

Z6 

Mar.  20 

4 

0 

8 

18.8 

0 

1 

92 

900 

2078 

260 

1068210 

At 

Mar.  30 

1 

0 

8 

16.5 

0 

0 

90 

516 

1962 

245 

717912 

B« 

April  6 

7 

0 

9 

15 

0 

0 

112 

323 

2092 

232 

529828 

C. 

April  13 

11 

0 

10 

17.5 

2 

0 

150 

515 

3309 

331 

1258685 

D« 

April  20 

3 

0 

11 

16 

7 

0 

110 

855 

3373 

307 

1759685 

E, 

April  28 

2 

0 

8 

19.3 

1 

1 

90 

900 

2924 

366 

1754176 

F, 

May  6 

1 

0 

13 

17.9 

0 

0 

75 

261 

1968 

151 

330808 

34  SELECTION   IN    CLADOCERA    ON   THE    BASIS   OF 

TABLE  4. — Summary  of  selection  data  by  broods  for  Line  695  minus — Continued. 


G 

2 

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A 

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G 

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3  8 

1  8 

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S  £ 

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K 

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80 

1915 

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May  13 

4 

0 

11 

17.8°C. 

3 

0 

70 

436 

2083 

189 

563453 

H, 

May  22 

1 

0 

8 

17.3 

0 

1 

90 

900 

2060 

258 

1114000 

le 

May  31 

3 

0 

6 

15.3 

0 

0 

246 

642 

2169 

362 

894821 

J« 

June  8 

3 

0 

10 

18.8 

0 

0 

220 

792 

4304 

430 

2259638 

K, 

June  15 

2 

0 

11 

20.6 

7 

0 

171 

395 

3033 

276 

895395 

Lt 

June  21 

3 

0 

6 

19.7 

0 

2 

290 

900 

3355 

559 

2293575 

M 

June  30 

2 

0 

6 

19.8 

1 

2 

330 

900 

3984 

664 

2920518 

N, 

July  7 

2 

0 

3 

21.9 

0 

0 

620 

665 

1937 

646 

1251729 

O« 

July  14 

Random  distribution. 

P'a 

July  20 

Random  distribution. 

July  28 

Random  distribution. 

Pa'* 

Aug.  7 

Random  distribution. 

Q< 

Aug.  13 

Random  distribution. 

Z 

Aug.  19 

Random  distribution. 

S, 

Aug.  30 

Random  distribution. 

T« 

Sept.  7 

5 

0 

7 

22.5 

1 

3 

150 

900 

4670 

667 

3614500 

Ue 

Sept.  13 

4 

0 

7 

20.9 

0 

0 

150 

630 

2000 

286 

740650 

V, 

Sept.  20 

4 

1 

20 

20.2 

0 

6 

130 

900 

12635 

632 

9572475 

Wi 

Sept.  29 

5 

0 

11 

18.2 

1 

2 

125 

900 

5660 

515 

3778100 

X, 

Oct.   9 

12 

1 

11 

16.5 

3 

0 

70 

450 

2070 

188 

514750 

Y» 

Oct.  18 

5 

0 

12 

18.4 

1 

0 

90 

690 

3445 

287 

1323225 

Z< 

Oct.  26 

5 

0 

18 

16.5 

3 

0 

70 

720 

4835 

269 

2095125 

AT 

Nov.  2 

4 

0 

12 

14.9 

0 

0 

60 

220 

1400 

117 

192650 

B; 

Nov.  9 

4 

0 

16 

14.8 

0 

0 

90 

435 

3200 

200 

751700 

CT 

Nov.  17 

4 

0 

11 

12.6 

2 

1 

100 

900 

3220 

293 

1479650 

DT 

Nov.  24 

4 

0 

17 

14.6 

0 

0 

65 

240 

2140 

126 

322050 

ET 

Dec.  3 

5 

0 

12 

12.3 

2 

0 

75 

510 

2120 

177 

526650 

FT 

Dec.  10 

5 

0 

11 

12.1 

0 

0 

65 

405 

1575 

143 

323025 

GT 

Dec.  17 

4 

0 

21 

20.2 

6 

1 

80 

900 

7580 

361 

3569200 

HT 

Dec.  25 

5 

1 

9 

11.9 

5 

1 

120 

900 

2820 

313 

1456550 

1916 

IT 

Jan.  4 

5 

0 

10 

14.5 

0 

10 

900 

900 

9000 

900 

8100000 

JT 

Jan.  13 

8 

1 

8 

18 

0 

1 

80 

900 

2280 

285 

1279400 

KT 

Jan.  23 

3 

1 

9 

19 

1 

0 

105 

600 

2115 

235 

673725 

LT 

Jan.  31 

9 

1 

11 

13.1 

1 

4 

130 

900 

5870 

534 

4265500 

MT 

Feb.  7 

4 

1 

17 

15 

0 

1 

50 

900 

3865 

227 

1580075 

NT 

Feb.  17 

8 

0 

32 

18 

0 

1 

50 

900 

5230 

163 

1562800 

OT 

Feb.  25 

2 

0 

12 

13.3 

0 

0 

80 

330 

2065 

172 

445075 

PT 

Mar.  4 

4 

0 

9 

12.1 

0 

0 

90 

310 

1705 

189 

373125 

Q7 

Mar.  13 

3 

1 

11 

18 

6 

0 

90 

750 

3845 

350 

1912925 

RT 

Mar.  20 

5 

1 

9 

15 

1 

1 

50 

900 

1865 

207 

1000125 

ST 

Mar.  27 

4 

1 

10 

15.2 

2 

0 

170 

840 

5105 

511 

3218875 

TT 

Apr.  8 

2 

0 

9 

14 

0 

0 

80 

540 

1840 

204 

624550 

UT 

Apr.  17 

9 

0 

15 

15.1 

0 

2 

75 

900 

5145 

343 

2648775 

VT 

Apr.  29 

4 

0 

5 

15.2 

0 

2 

180 

900 

2465 

493 

1771525 

W7 

May  9 

4 

1 

13 

16.7 

3 

0 

70 

500 

3180 

245 

1041000 

XT 

May  17 

3 

0 

10 

16.8 

0 

2 

70 

900 

2800 

280 

1783150 

YT 

June  5 

5 

0 

5 

19 

0 

0 

145 

720 

2150 

430 

1141050 

ZT 

June  13 

5 

0 

6 

17.8 

1 

0 

100 

600 

1390 

232 

493050 

As 

June  20 

4 

0 

13 

17.8 

1 

0 

75 

420 

2020 

155 

412700 

B, 

June  30 

4 

0 

12 

18.8 

0 

0 

80 

360 

2010 

168 

403300 

Cs 

July  7 

6 

0 

7 

19.5 

0 

3 

180 

900 

3970 

567 

2889700 

D, 

July  14 

3 

1 

6 

20.7 

2 

0 

135 

470 

1655 

276 

521025 

E8 

July  21 

3 

0 

5 

21.1 

0 

4 

270 

900 

3870 

774 

3312900 

F, 

Aug.  3 

Random  distribution. 

Gs 

Aug.  14 

5 

1 

26 

19 

0 

0 

110 

460 

4100 

168 

869400 

H, 

Aug.  19 

12 

0 

7 

19.8 

0 

0 

120 

360 

1495 

213 

360025 

Is 

Aug.  26 

2 

0 

21 

21 

0 

1 

80 

900 

8540 

407 

4770700 

Js 

Sept.  1 

2 

0 

11 

19 

2 

0 

160 

760 

4420 

402 

2220000 

•Strain  lost  July  22  and  renewed  from  substrain 
eloaet  for  37  generations. 


of  695  -  (695  Mi  -)  which  hud  been  kept  in   a  dark 


A   PHYSIOLOGICAL   CHARACTER. 


35 


a  better  treatment  of  the  data  is  secured  by  considering  it  by  unit 
groups  larger  than  a  single  brood.  Convenient  units  of  comparison 
are  month  and  two-month  periods,  and  summaries  of  the  data  by 
such  periods  will  be  referred  to  rather  than  the  data  by  broods. 

Tables  5  and  6  give,  in  successive  columns,  the  two-month 
periods;  the  generations  of  descent  during  the  periods;  the  number  of 
broods  used  in  selection;  the  total  number  of  young  tested;  the 
average  number  of  individuals  per  brood;  the  average  age  of  the 
mothers  at  the  time  their  first  broods  were  produced;  the  average 
number  of  young  per  day  of  the  mother's  age  at  the  time  of  produc- 
tion of  the  first  brood — the  reproductive  index;  the  total  number  of 
individuals  moving  to  the  negative  end  of  the  tank;  the  number  of 
individuals  failing  to  reach  either  end  of  the  tank  during  the  time  of 
the  test  (15  minutes);  the  average  minimum  reaction- time  for  the 
different  broods;  the  average  maximum  reaction-time;  the  sum  of 
the  individual  reaction-times  (for  computation  purposes) ;  the  mean 

TABLE  5. — Selection  data  summarized  by  two-month  periods  for  Line  695  plus. 


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13760 

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240.92 

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Aug.-Sept.  1912.  .  . 

20-23 

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Oct.-Nov.  1912.  .  . 

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Dec.  1912-  Jan.  1913 

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13761 

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Feb.-Mar.  1913..  . 

36-42 

7 

56 

8.0 

8.3 

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0 

3 

158 

447 

16930 

302 

Apr-May   1913... 

43-50 

8 

67 

8.4 

7.8 

1.08 

1 

0 

100 

331 

13345 

199 

June-  July   1913.  .  . 

51-58 

9 

35 

3.9 

7.2 

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3 

5 

209 

462 

12355 

353 



Aug.—  Sept.  1913... 

59—67 

9 

74 

7.2 

6.7 

1.07 

2 

3 

114 

548 

20737 

280 

Oct.-Nov.  1913. 

68-75 

66 

8A 

7.5 

1.12 

2 

1 

133 

474 

16019 

243 

Dec.  1913—  Jan.  1914 

76—83 

7 

50 

7  .0 

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4 

7 

253 

822 

25253 

505 

Feb.-Mar.  1914. 

84-89 

56 

9.3 

8.3 

1.12 

4 

10 

158 

634 

24443 

436 

Apr.—  May    1914... 

90—98 

g 

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8^2 

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1.  15 

18 

15 

198 

766 

32718 

436 

June-July    1914.'.'! 

99-105 

7 

40 

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6 

2 

149 

693 

15033 

376 

224.64 

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Aug.—  Sept.  1914... 

106—114 

9 

97 

8.2 

6.6 

1.24 

8 

18 

152 

783 

43400 

447 

Oct  -Nov    1914 

115—122 

8 

106 

12  5 

7.5 

1  67 

27 

4 

125 

776 

35712 

337 

Dec.  1914-Jan.  1915 

123-130 

8 

66 

8^4 

7.1 

1.18 

8 

7 

111 

612 

20726 

314 

Feb.-Mar.  1915  

131-138 

7 

85 

11.6 

8.3 

1.40 

5 

4 

114 

664 

26122 

307 

Apr.-May    1915... 

139-145 

8 

81 

10.0 

8.6 

1.16 

13 

11 

180 

876 

34741 

429 

279  '.  77 

'26:97 

June-July    1915... 

146-153 

5 

43 

10.5 

7.9 

1.33 

4 

5 

216 

723 

15561 

362 

267.64 

27.53 

Aug.-Sept.  1915... 

154-160 

4 

26 

6.6 

9.2 

.72 

8 

3 

138 

566 

9530 

367 

246.41 

32.60 

Oct.-Nov.  1915.  . 

161-166 

7 

107 

17.7 

8.4 

2.11 

5 

2 

74 

662 

25300 

236 

Dec.1915-.Jan.  1916 

167-174 

7 

80 

12.4 

8.5 

1.46 

9 

5 

84 

643 

19710 

246 

206!  09 

'15.  54 

Feb.-Mar.  1916.  .. 

175-181 

6 

70 

10.3 

7.9 

1.30 

5 

5 

91 

664 

18570 

265 

Apr.-May    1916... 
June-July    1916... 

182-187 
188-195 

6 
6 

67 
49 

11.7 
9.0 

10.4 
7.2 

1.13 
1.25 

6 
6 

10 
12 

88 
163 

787 
612 

22090 
21200 

330 
433 

288.31 

27.78 

Aug.-Sept.  1916... 

196-200 

G 

89 

9.7 

6.6 

1.47 

6 

30 

173 

900 

42430 

477 

338.61 

24.21 

•  Including  all  the  data  available,  some  of  which  is  additional  to  that  for  the  broods  tested. 
bAll  available  data  is  here  used,  including  that  for  the  No.  2  mothers  whose  broods  were  not  ordinarily 
tested  for  use  in  selection. 


36 


SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 


individual  reaction-time;  the  standard  deviation  of  this  mean;  and 
its  probable  error. 

For  the  minus  strain,  table  6  gives  three  additional  columns:  the 
difference  between  the  means  for  the  plus  and  the  minus  strains ;  the 
probable  error  of  this  difference;  and  the  quotient  of  this  difference 
divided  by  the  probable  error. 

These  tables  are  divided  into  sections,  each  containing  the  data 
for  a  period  of  a  year,  except  the  data  obtained  previous  to  August 

TABLE  6. — Selection  data  summarized  by  two-month  periods  for  Line  695  minus. 


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437 

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Dec.  1913-Jan.  1914 

71-77 

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Feb.-Mar.  1914  .  .  . 

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413 

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June-July   1914... 

93-99 

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29 

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9.8 

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7 

6 

374 

670 

13478 

465 

170.94 

21.41 

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32.13 

2.76 

Aug.-Sept.  1914... 

100-107 

8 

82 

9.6 

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33 

22 

11 

176 

762 

32288 

394 

+  53 

Oct.-Nov.  1914  .  .  . 

108-116 

9 

123 

12  4 

7.0 

.77 

28 

3 

94 

648 

34167 

278 

+  59 

...  * 

Dec.  1914-Jan.  1915 

117-124 

7 

74 

8.7 

8.1 

.07 

9 

4 

105 

605 

22389 

303 

+   11 

Feb.-Mar.  1915.  .. 

125-131 

S 

84 

11.0 

8.1 

.36 

6 

4 

88 

628 

21205 

252 

+  55 

Apr-May   1915... 

132-139 

8 

76 

9.9 

7.6 

.30 

13 

2 

118 

604 

9978 

263 

97  23 

5^26 

+  166 

25!  93 

5  40 

June-July    1915... 

140-145 

5 

36 

8.7 

7.6 

.14 

8 

4 

326 

730 

6613 

461 

233^14 

26.21 

-  99 

38.01 

2.'eo 

Aug.-Sept.  1915.  .  . 

146-153 

4 

45 

8.8 

8.4 

1.05 

2 

11 

139 

833 

24965 

555 

292.86 

29.45 

-188 

43.93 

4.27 

Oct.-Nov.  1915... 

154-160 

7 

97 

14.1 

8.0 

1.76 

9 

1 

78 

522 

20310 

209 

+  27 

Dec.  1915-Jan.  1916 

161-168 

8 

91 

12.1 

8.4 

1.44 

15 

17 

194 

752 

33360 

367 

95  .'95 

26!  93 

-121 

26.'  06 

4.'64 

Feb.-Mar.  1916... 

169-175 

7 

100 

11.3 

7.6 

1.49 

9 

3 

83 

704 

23680 

237 

+   28 

Apr.-May    1916... 

176-181 

5 

52 

11.0 

11  6 

.95 

3 

6 

95 

748 

5430 

297 

+  33 

June-July   1916... 

182-188 

7 

54 

6.5 

8.6 

.76 

4 

7 

141 

624 

7065 

316 

64.62 

24.29 

+  117 

36.90 

3.17 

Aug.-Sept.  1916... 

189-195 

4 

65 

8.8 

7.3 

1.21 

2 

1 

118 

620 

8555 

285 

212.15 

7.75 

+  192 

30.02 

6.39 

•  Including  all  the  data  available,  some  of  which  is  additional  to  that  for  the  broods  tested. 

b  All  available  data  is  here  used,  including  that  for  No.  2  mothers  whose  broods  were  not  ordinarily  tested  for  use  in  selection. 

1,  1912,  and  subsequent  to  July  31,  1916.  The  data  obtained  previ- 
ous to  August  1,  1912,  deserve  less  serious  consideration  than  may 
be  given  the  data  obtained  later,  since  previous  to  that  time  improved 
methods  of  handling  the  material  and  additional  experimental  pre- 
cautions were  occasionally  being  devised  and  utilized.  From  August 
1,  1912,  the  methods  of  handling  the  Cladocera  material  were  pretty 
well  standardized  and  there  seems  every  reason  to  believe  that  the 
data  are  such  as  would  be  obtained  by  repetition  of  the  experiments. 


A   PHYSIOLOGICAL   CHARACTER. 


37 


The  mean  reaction-times  obtained  in  the  tabulation  by  two- 
month  periods  (tables  5  and  6)  are  used  in  the  plotting  of  curves 
(figure  2c),  showing  graphically  the  courses  of  the  selection  experi- 
ment with  Line  695. 

Table  7  gives  in  abbreviated  form  selection  summaries  of  the 
data  for  695  plus  and  695  minus  by  the  larger  periods  (mostly  year- 
periods)  into  which  it  was  divided  in  tables  5  and  6;  it  also  includes 
summaries  and  results  of  the  "test  series"  and  summaries  for  selection 
data  for  three-month  periods  during  which  the  test  series  were  con- 
ducted. In  table  7,  in  the  vertical  column  to  the  left,  are  indicated 

TABLE  7. — Selection  summary  for  Line  695. 


<o 
I 

, 

individuals  1 
se  tested. 

dduals  test-  1 
lections. 

reacting  in- 

fl 

n  reaction-  1 

n  reaction-  1 

I  reaction- 

d 

i 

i 

difference. 

! 

V 

« 

"o  -§ 

%  8 

>> 

1? 

3 

3 

1 

B 

"8 

S 

0 

1 

Time  period. 

Strain. 

"3 

§ 

| 

oo 
fc-g 

.2  «« 

55  ^ 

1 

0)  JH 

3§ 

1 

a 

individ 

I 

J 

|| 
It 

1 

1 

"8 

fe  •- 

-j.S 

i! 

"s-g 

li 

li 

ti   § 

1 

1 

Sl 

1 

i  « 

0 

1 

4& 

g"8 

6  S 

4- 

£.2 

Q)    Q 

1 

1 

HI 

1 

Stl  £ 

Apr.     1,  1912- 
July  31,  1912 

(  Plus  .... 
\  Minus  .  . 

8-19 
8-19 

13 

IS 

4.0 

4.7 

51 

61 

4 

9 
18 

302 
2S1 

708 

734 

497.3 
433.1 

259.2 
308.  S 

24.49 

26.62 

-  1.8 

36.17 

0.06 

Aug.    1,  1912- 

{  Plus  .... 

20-58 

42 

6.0 

251 

4 

10 

148 

403 

260.2 

182.5 

7.77 

July  31,  1913 

(  Minus  .  . 

19-54 

SS 

5.7 

217 

8 

1S5 

472 

297.6 

187.7 

8.69 

'-37.3 

ll'.69 

s'.'f 

Aug.    1,  1913- 

(  Plus.  ... 

59-105 

46 

7.8 

361 

36 

38 

166 

653 

371.8 

253.3 

8.99 

+  0.5 

13.10 

0.04 

July  31,  1914 

\  Minus  .  . 

65-99 

42 

7.1 

304 

31 

31 

231 

668 

371.3 

246.5 

3.54 

Test   series, 

(  Plus.  ... 

61 

24 

22.0 

529 

44 

133 

181 

769 

522.2 

284.4 

8.34 

+31.7 

12'  i4 

2'  6i 

Aug.,  1913... 

(  Minus  .  . 

57 

24 

13.5 

28 

103 

171 

£37 

490.6 

282.6 

8.82 

July     1,  1913- 

(  Plus  

55-67 

10 

7.2 

72 

2 

4 

169 

536 

296 

+  2 

Sept.30,  1913 

I  Minus  .  . 

61-64 

12 

7.5 

30 

1 

5 

206 

657 

234 

Test   series, 

(  Plus  

95 

51 

24.3 

1083 

89 

290 

114 

863 

501 

3ii!4 

6.36 

May,  1914  .  . 

t  Minus  .  . 

89 

fil 

24-4 

1088 

76 

168 

852 

562.  S 

302.2 

6.19 

-61  '.8 

'8.88 

6.  96 

Apr.    1,  1914- 

(  Plus  .... 

90-100 

11 

7.5 

83 

20 

16 

194 

786 

440.2 

June  30,  1914 

V  Minus  .  . 

86-95 

10 

6.2 

62 

17 

10 

712 

441.2 

—  1.0 

Aug.    1,  1914- 

<  Plus  .... 

106-153 

45 

10.7 

478 

65 

49 

146 

743 

368.7 

225.4 

6.97 

+60.0 

'ioies 

5.  62 

July  31,  1915 

I  Minus  .  . 

100-147 

45 

10.5 

475 

S6 

28 

140 

653 

308.7 

262.3 

8.09 

Test   series, 

/  Plus.  .  , 

142 

24.3 

961 

122 

67 

108 

900 

350.1 

222.9 

4.85 

May,  1915  .  . 

\  Minus  .  . 

109 

24.4 

374 

110 

57 

133 

731 

351.2 

216.2 

4.67 

-  '  1  '.  1 

6.73 

0.16 

Apr.     1,  1915- 

I  Plus  .... 

139-149 

12 

10.0 

120 

16 

16 

168 

829 

399.2 

279.4 

17.20 

+81.3 

22.56 

June  30,  1915 

I  Minus  .  . 

132-143 

12 

3.1 

103 

Sl 

6 

163 

652 

317.3 

225.8 

14.69 

Test  series, 

(  Pius.... 

143 

16 

15.6 

249 

43 

29 

128 

747 

369 

244.3 

10.43 

+si!i 

14.08 

3.63 

June,  1915  .  . 

I  Minus  .  . 

143 

16 

15.4 

246 

26 

16 

117 

755 

317.3 

220.  1 

3.46 

May    1,  1915- 

i  Plus  .... 

143-153 

8 

8.4 

67 

10 

7 

221 

772 

397.9 

264^5 

21.80 

+  61.5 

28  !  63 

2'.i4 

July  31,  1915 

\  Minus 

136-147 

# 

3.3 

74 

11 

C 

235 

655 

336.4 

236.8 

18  57 

Aug.    1,  1915- 

f  Plus  .  .  .  .' 

154-195 

36 

11.1 

399 

31 

34 

103 

635 

291.5 

242.5 

8.19 

July  31,  1916 

(  Minus  .  . 

148-187 

88 

11.6 

685 

307.1 

262.8 

8.46 

-Id'.  6 

11  77 

I'.SS 

Aug.    1,  1916- 

(  Plus  .... 

196-203 

8 

12.5 

102 

6 

30 

156 

732 

436.3 

338.6 

24.21 

+150.8 

30^2 

5.0 

Sept.  2,  1916 

I  Minus  .  . 

188-192 

4 

16.3 

2 

1 

118 

620 

285.6 

212.1 

17.74 

the  periods  covered  by  the  various  portions  of  the  summary;  succes- 
sive columns  to  the  right  of  this  indicate  the  strain  (whether  plus  or 
minus);  the  generations  of  descent  during  the  period  under  con- 
sideration; the  number  of  broods  used  in  selection  and  in  obtaining 
the  data;  the  average  number  of  individuals  per  brood  of  those 
tested;  the  total  number  of  individuals  tested  in  making  selections; 
the  number  of  individuals  moving  to  the  negative  end  of  the  tank; 
the  number  failing  to  respond  to  the  extent  of  reaching  either  end  of 
the  tank;  the  average  minimum  reaction-time;  the  average  maximum 
reaction- time ;  the  mean  individual  reaction-time;  standard  deviation 


38 


SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 


of  this  mean;  its  probable  error;  the  difference  between  the  mean  for 
the  plus  strain  and  that  for  the  minus  strain;  the  statistical  probable 
error  of  this  difference ;  and  the  quotient  of  this  difference  divided  by 
its  statistical  probable  error.  For  each  period  considered  in  this 
table  the  corresponding  data  for  the  plus  and  minus  strains  are  given 
in  successive  horizontal  columns,  the  plus  first  and  the  minus  in 
italics  immediately  below  it.  Thus  the  items  for  making  the  signifi- 
cant comparisons  between  the  plus  and  minus  strains  are  found 
together. 

Table  8  gives  a  summary  of  the  data  for  the  " same-day"  broods, 
i.  e.,  those  broods  of  the  mothers  of  the  corresponding  plus  and  minus 
strains  which  reproduced  on  the  same  days  and  the  young  of  which, 
consequently,  were  tested  under  identical  conditions.  The  arrange- 
ment of  the  data  is  similar  to  that  for  table  7. 

TABLE  8. — Same-day  broods.    Summary  of  data  for  Line  695. 


I 

h 

3 

i 

g 

i 

c 

I 

8 

S 

| 

af  young 

"o 

OJ 

11 

«n3 

1 

1 

1 

0 

c 

mean. 

n  means 
strains. 

• 

1 

! 

Time  period. 

Strain. 

| 

e  number 

individual 

negatively 
lals. 

individua 
an  end  of 

e  minimu 

e  maximu 

individua 

•3 

I 

"8 
1 

nee  betwet 
nd  minus 

I 

i 

•o 

2  = 

"5 

"S3 

"o  "S 

2g 

2  « 

c  a5 

i 

I 

fe  « 

J 

i  « 

^0 

1 

1- 

o  o 

S* 

1- 

03 
02 

1 

s3- 

.2 

a= 

Apr.    6,  1912- 

(  Plus.  .  .  . 

a 

8  5 

17 

3 

4 

193 

QOO 

545.3 

July  31,  1912 

\  Minus 

0 

7  5 

15 

5 

360 

500 

633  0 

—   88 

Aug.     1,  1912- 

(  Plus  .... 

]  } 

4  3 

47 

2 

7 

173 

455 

332.9 

July  31,  1913 

I  Minus.  .  . 

11 

5.6 

62 

4 

ISO 

60J 

333.7 

* 

Aug.    1,  1913- 

f  Plus  .  .  . 

7  1 

64 

1 

3 

138 

5?7 

290.6 

July  31,  1914 

(  Minus 

() 

8  1 

73 

^ 

60S 

30#  9 

—    12 

Aug     1,  1914- 

[  Plus 

n 

10  5 

63 

6 

2 

125 

662 

314  2 

4-      2 

July  31,  1915 

(  Minus.  .  . 

G 

11.  S 

68 

17 

3 

67.9 

311.9 

Aug     1,  1915- 

<  pjus 

10 

12  0 

120 

15 

g 

92 

622 

284  0 

223  11 

13  74 

July  31,  1916 

(  Minus.  .  . 

10 

14.6 

146 

5 

9 

677 

277.1 

250.74 

14-00 

—     7 

19.61 

0.35 

Aug.     1,  1916- 

i  Plus  

3 

18.3 

55 

2 

12 

110 

900 

383.6 

324.97 

29.56 

+  122 

35.10 

3.47 

Sept.  2,  1916 

'  Minus.  .  . 

3 

18.0 

54 

0 

103 

573 

261.8 

206.  SS 

18.94 

Figure  2c  shows  in  diagrammatic  form  the  mean  reaction-times 
of  the  plus  and  minus  strains  of  Line  695  by  two-month  periods. 
The  distances  from  the  axis  of  ordinates  represent  two-month  periods 
of  the  selection  data.  The  ordinates  indicate  the  average  reaction- 
times  in  seconds  for  the  two-month  periods.  The  solid  line  represents 
the  course  of  the  averages  for  the  plus  strain,  the  broken  line  for  the 
minus  strain.  The  roman  numerals  indicate  the  times  at  which  the 
test  series  were  conducted,  and  the  plus  and  minus  signs  within  the 
diagram  indicate  the  positions  of  the  means  for  these  test  series. 

ANALYSIS  OF  DATA  FOR  EFFECT  OF  SELECTION. 

On  the  whole,  there  is  a  marked  coincidence  of  the  fluctuations 
of  the  two  strains,  the  means  for  the  plus  and  minus  strains  in  a 
general  way  following  the  same  upward  and  downward  trends.  This 


A   PHYSIOLOGICAL   CHARACTER.  39 

is  obviously  due  to  environmental  factors.  The  data  for  the  first  4 
months  previous  to  August  1,  1912,  may  be  passed  over  with  slight 
consideration,  for  (as  stated  above)  the  methods  of  handling  the 
material  were  not  sufficiently  standardized  at  first.  These  first  2 
two-month1  periods  show  rather  widely  different  averages  (see  tables 
6  and  7  and  figure  2c),  but  from  this  time  on  less  violent  fluctuations 
in  average  reaction-times  are  encountered,  except  for  very  wide 
fluctuations  in  the  average  of  the  minus  strain  during  the  latter  half 
of  1915,  and  an  unusual  upshoot  in  the  plus  strain  during  the  last  4 
months  of  the  experiment.2 

For  the  year,  August  1,  1912- July  31,  1913,  the  mean  reaction- 
time  for  the  plus  strain  was  260.2  seconds  (251  individual  reaction- 
time  records;  see  table  7)  and  for  the  minus  strain  297.5  seconds 
(217  individual  reaction-time  records).  The  minus-strain  mean  was 
—  37.3  seconds  greater  than  that  for  the  plus  strain.  The  difference 
was  3.2  times  the  statistical  probable  error  (table  7).  This  large 
difference  is  due,  of  course,  to  the  difference  obtaining  for  the  first 
6  months  of  this  year-period.  This  six-month  period  is  discussed 
later.  In  the  latter  half  of  the  year  there  was  not  a  consistency  in 
reaction-time  differences,  though  on  the  average  the  plus  was  slightly 
the  more  reactive.  The  "same-day"  broods  for  the  year-period  have 
a  mean  reaction-time  for  the  minus  strain  only  0.8  second  greater 
than  for  the  plus  strain  (table  8). 

For  the  year,  August  1913-July  1914,  the  curves  show  that  the 
two  strains  differ  little  in  mean  reaction-time  (plus  mean  371.8 
seconds  with  361  individual  reaction- times ;  minus  mean  371.3 
seconds  with  304  individual  reaction-times;  see  table  7)  and  in  relative 
vigor  (see  figure  2s).  For  the  entire  year  the  mean  reaction-times 
differ  by  only  +0,5  second  and  the  same-day  broods  by  only  —12.3 
seconds  (table  8).  Two  test  series  were  conducted  during  this 
period.  One  in  August  1913  consisted  of  529  individuals  in  the  plus 
strain  and  467  individuals  in  the  minus  strain.  The  mean  reaction- 
times  were  522.2  seconds  and  490.5  seconds  for  the  plus  and  minus 
strains  respectively,  a  difference  of  +31.7  seconds  ±12.14  seconds. 
This  difference  is  2.61  times  the  probable  error.  The  second  test 
series,  May  1914,  consisted  of  1,083  and  1,088  individuals  and  the 
means  were  501  and  562.8  seconds.  The  difference  in  seconds  was 
-61.8 ±8.88,  a  difference  6.96  times  the  probable  error.  The 
results  of  these  two  test  series  are  contradictory  and  judgment  will 
be  reserved  until  the  later  data  are  considered. 

1  The  summaries  of  the  data  for  the  earlier  portion  of  the  experiments  were  made  to  include 
the  first  4  months.    Summaries  thereafter  were  made  covering  year  periods.    Summaries  for  the 
lines  later  subjected  to  selection  were  made  to  conform  to  the  summary  periods  already  utilized 
for  the  older  lines. 

2  Actually  this  is  only  a  trifle  more  than  3  months,  as  the  selection  was  discontinued  Septem- 
ber 2,  1916. 


40        SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

The  year,  August  1914-July  1915,  gives  as  an  average  for  the 
plus  strain  368.7  seconds  (478  individual  records),  60  seconds  greater 
than  that  for  the  minus  strain  (475  individual  records),  a  difference 
5.62  times  the  statistical  probable  error.  For  the  first  10  months  the 
plus  strain  consistently  showed  a  higher  reaction-time.  For  the  6 
same-day  broods  the  plus  strain  had  a  higher  reaction-time  by  2.3 
seconds.  A  test  series  (May  1915,  table  7)  showed  a  higher  reaction- 
time  for  the  minus  strain,  but  the  difference  was  only  —1.1  ±6. 73. 
Another  test  series  (June  1915)  gave  a  higher  reaction-time  by 
51.1  ±14. 08  seconds  for  the  plus  strain.  This  difference  was  3.63 
times  the  probable  error. 

During  the  remainder  of  the  experiment  with  Line  695  (August 
1915-September  1916),  the  minus  strain  fluctuated  rather  widely 
in  mean  reaction-time  (figure  2c).  For  the  year  ending  July  31, 
1916  (399  and  439  individual  reaction-time  records),  the  minus 
strain  had  the  higher  reaction-time  by  15. 6  ±11. 77  seconds.  The 
same-day  broods  gave  a  difference  of  —  6. 9  ±19. 61  seconds.  The 
remaining  short  period  of  the  experiment,  a  trifle  more  than  a  month, 
shows  a  considerably  higher  reaction-time  (higher  by  150.8  seconds) 
for  the  plus  strain. 

If  the  experiment  with  Line  695  had  been  discontinued  in 
January  1913,  one  might  have  felt  inclined  to  ascribe  a  possible  effect 
of  selection  to  the  strains  of  this  line  (see  figure  2c).  But  it  is 
extremely  improbable  that  such  an  effect  really  should  be  thus 
ascribed  for  that  period  and  that  the  selective  effect  once  obtained 
was  later  lost  in  some  manner.  If  one  were  to  interpret  this  as  an 
effect  of  selection  acquired  through  a  mutation  or  acquired  in  some 
other  manner,  it  seems  quite  difficult  to  account  for  its  loss.  The 
selections  were  not  relaxed  and  there  was  no  period  of  high  mortality 
among  the  stock  to  account  for  the  loss  of  a  selective  difference.  In 
two  portions  of  the  experiment  the  plus  strain  consistently  had  a 
higher  reaction-time  for  8  months  or  more  at  a  time.  Considering 
the  curves  as  a  whole,  there  are  three  periods  in  which  one  or  the 
other  strain  had  a  higher  reaction-time  for  as  long  as  6  months  at  a 
time.  In  two  of  these  periods  the  plus  strain  had  the  higher  re- 
action-time and  in  one  of  the  minus  had  the  higher  reaction-time 
A  further  examination  of  these  data  will  be  made  in  order  to  seek 
the  explanation. 

RELATION  BETWEEN  REPRODUCTIVE  VIGOR  AND  REACTIVENESS  TO  LIGHT. 

It  would  seem  plausible  to  suppose  that  relatively  slight  differ- 
ences in  the  general  vigor  of  the  two  strains  might  be  a  factor  in 
determining  the  reaction-time,  particularly  in  such  periods  as  those 
just  referred  to. 

The  rate  of  descent  of  a  strain  may  be  considered  one  measure 
of  its  general  vigor.  Figure  2A  presents  the  relative  rates  of  descent 
of  the  two  strains  of  Line  695;  the  base-line  represents  equality  of 


A   PHYSIOLOGICAL   CHARACTER.  41 

descent,  i.  e.,  if  mothers  of  the  same  generation  of  descent  in  the 
two  strains  produce  their  young  on  the  same  day  the  circle  will  fall 
upon  the  base-line  with  the  figure  0  adjoining.  If  young  from  the 
same  generation  of  descent  appeared  13  days  later  in  the  plus  than 
in  the  minus  strain,  the  solid  circle  will  fall  on  the  base-line  with 
a  figure  13  placed  above  the  line,  while  if  the  minus  strain  (as  in  the 
case  of  the  seventh  entry  in  this  diagram)  were  4  generations  and  a 
day  behind  the  plus  strain  in  producing  young  of  a  given  generation, 
this  is  indicated  by  the  open  circle  being  placed  4  units  below  the 
base-line,  with  a  figure  1  adjoining. 

By  comparing  the  relative  amounts  of  descent  of  the  two  strains 
from  the  end  of  one  two-month  period  to  the  end  of  the  following 
two-month  period,  one  can  determine  whether  the  plus  or  the  minus 
strain  had  during  that  interval  descended  the  more  rapidly,  i.  e., 
which  strain  was  presumably  the  more  vigorous  of  the  two  during 
that  limited  period.  For  example,  from  the  end  of  May  1912  (first 
entry  in  this  diagram)  to  the  end  of  July  1912  (second  two-month 
period  and  second  entry  in  the  diagram),  the  minus  strain  from 
having  been  1  generation  and  2  days  in  advance  of  the  plus  strain 
had  become  13  days  ahead  of  the  plus  strain  in  its  time  of  producing 
young  of  the  same  generation  of  descent.  During  the  following  two- 
month  period  (the  third)  the  minus  strain  became  just  1  generation 
(third  entry  in  the  diagram)  behind  the  plus  strain.  Still  one  period 
later  (the  fourth)  it  was  1  generation  and  7  days  in  arrears  of  the 
plus  strain. 

Comparing  rate  of  descent  with  mean  reaction-time,  the  minus 
strain  during  the  second  two-month  period  descended  the  less  rapidly, 
but  was  the  more  reactive  of  the  two  strains.  During  the  third 
period  the  minus  strain  descended  the  less  of  the  two  strains  and  was 
the  less  reactive.  During  the  fourth  period  the  minus  strain  again 
descended  the  less  rapidly  and  was  the  less  reactive.  While  the 
difference  in  reaction-time  is  frequently  in  the  direction  expected  if 
influenced  or  determined  by  the  relative  rates  of  descent  of  the  two 
strains,  such  is  not  true  more  often  than  the  reverse  is  true.  For 
the  27  two-month  periods  of  selection  with  Line  695,  the  more 
rapidly  descending  of  the  two  strains  had  the  lower  reaction-time 
in  10  cases,  the  higher  reaction-time  in  13  cases;  for  3  two-month 
periods  their  rates  of  descent  were  identical;  and  for  1  period  their 
mean  reaction-times  were  identical.  That  is,  the  more  vigorous 
strain,  judged  by  its  rate  of  descent,  was  the  less  reactive  during  13 
of  the  two-month  periods,  and  in  fewer  cases  (10)  the  more  vigorous 
strain  was  the  more  reactive  one.  Hence,  judged  by  this  measure 
alone,  the  more  vigorous  strain  was  the  more  reactive  strain  less 
often  than  it  was  the  less  reactive  strain. 

In  a  further  endeavor  to  learn  if  relative  vigor  is  a  factor  in 
influencing  reaction-times,  all  of  our  data  for  Line  695  which  seemed 


42  SELECTION    IN    CLADOCERA    ON    THE    BASIS    OF 

to  bear  on  the  general  vigor  of  the  stock  were  tabulated.  In  con- 
sidering this  point  three  criteria  of  the  general  vigor  of  the  material 
were  used,  the  age  of  the  mother  at  the  time  her  first  brood  was 
produced,  the  number  of  young  in  her  first  brood,  and  the  interval 
between  the  first  and  second  broods.  From  a  general  knowledge  of 
the  material  and  use  in  other  connections  of  these  measures  of  the 
vigor  of  the  material,  the  writer  can  state  that  these  are  satisfactory 
criteria.  Figure  IA  shows  graphically  the  average  number  of  young 
in  the  first  brood  for  Line  695  by  the  two-month  periods  of  the 
experiment.  Figure  IB  shows  the  average  age  of  mother  for  the 
same  periods. 

1913  1913  1914  1915  1916 

4-5    6-9    12-1    4-5    8-9    12-1    4-5    8-9    12-1    4-5    8-9    12-1    4-5    8-9 


00 


5 

I 


10 


B 

!' $5S*8PSJ^8s** 


4-5         8-9         12-1         4-5        8-9         12-1         4-5         8-9         12-1        4-5         8-9   ^  12-1 

I 


1912  1913  1914  1915          T  1916 


C.  ,       *       T5U 


FIGURE  1. — Line  695.    Reproductive  vigor. 

A.  Average  number  in  the  first  brood  by  two-month  periods.    The  vertical  scaling  above 
the  base-line  indicates  the  number  of  young;  the  horizontal  scaling,  the  two-month  periods  of 
the  experiment.    Solid  circles  show  positions  for  the  plus  strain;  open  circles  for  the  minus  strain. 

B.  Average  age  of  mothers  at  time  first  brood  was  produced.     Vertical  scaling  below  the 
base-line  indicates  the  number  of  days. 

C.  Actual  values  of   reproductive  indices  (average  number  of  young  in  first  brood  divided 
by  average  age  of  mother  at  time  first  brood  was  produced).     Vertical  scaling  indicates  the 
numerical  values  of  the  reproductive  index.     Each  numerical  value  of  the  reproductive  index 
(in  C)  is  obtained  by  dividing  the  corresponding  numerical  value  in  A  by  the  corresponding 
numerical  value  in  B. 


-    2 


A   PHYSIOLOGICAL   CHARACTER. 

A 


43 


4-5        6-9 


o    o 
7     ' 


12-1 


&          oo 
3    2 


o    o 

44 


OOOOOO 


4-5        8-9 
'913 


12-1 


o    o          o    o    o    o 

22049S509020|OI00_ 

I  0 

4-5       8-9        12-1        4-5        8-9        12-1        4-5        8-9 
1914  1915  1916 


"t  T.T! 


.50 
.25 
00 
.25 
.50 


4-5       8-9 
1912 


12-1 


4-5        8-9 
1913 


12-1 


II 


4-5        8-9 
1914 


12-1 


m  rv 


4-5        8-9 
1915 


12-1 


4-5       8-9 
1916 


FIGURE  2. — Line  695. 

A.  Relative  rates   of  descent  of  the  two  strains.    The  base-line  represents  an  equality  of 
descent,  i.  e.,  if  both  strains  produced  young  of  the  same  generation  of  descent  on  the  same 
day  the  circle  would  fall  on  the  base-line  with  a  figure  "0"  placed  adjacent.    Positions  above 
the  base-line  (solid  circles)  indicate  that  the  minus  strain  in  a  particular  generation  was  lagging 
by  as  many  generations  as  the  point  falls  in  units  below  the  base-line  and  by  as  many  additional 
days  as  are  indicated  by  the  adjacent  numeral.    The  horizontal  scaling  indicates  the  two-month 
periods  of  the  experiment. 

B.  Reproductive  indices  plotted  to  show  which  strain  is  superior  in  vigor  (and  by  how  much 
it  is  superior)  in  the  different  two-month  periods  in  experiment.    Solid  circles  indicate  that  the 
plus  strain  is  superior  by  the  amount  indicated  by  the  distance  the  solid  circle  lies  above  the  base- 
line; conversely,  open  circles  below  the  base-line  indicate  the  amount  of  superiority  of  the  minus 
strain.    The  horizontal  scale  indicates,  as  always,  the  two-month  periods  of  the  experiment. 

C.  Reaction-time  curves.    The  plus  strain  in  solid,  the  minus  strain  in  broken  line.    The 
vertical  scale  indicates  seconds  of  reaction-time,  the  horizontal  scale  two-month  periods  of  the 
experiment.     The  points  in  the  curves  represent  mean  individual  reaction-times  by  two-month 
periods.    The  roman  numerals  indicate  times  at  which  "test  series"  were  conducted,  and  the 
plus  and  minus  signs  the  positions  for  the  test  series  means  for  the  two  strains. 

It  seemed  desirable  to  combine  these  measures  so  as  to  write  the 
result  in  a  single  quantitative  term.  This  was  in  part  accomplished 
by  dividing  the  mean  number  of  young  in  the  first  brood  by  the  mean 
age  of  the  mothers  at  the  time  the  first  broods  were  produced.  This 
gives  the  average  number  of  young  per  day  of  the  mother's  age  and 
may  be  called  the  reproductive  index.1  This  reproductive  index  is 

1  Unfortunately  this  did  not  include  data  for  the  interval  between  the  first  and  second 
broods.  This,  however,  was  the  least  important  of  the  three  available  measures  of  vigor,  and 
the  data  for  it  was  less  extensive  than  for  the  other  two  measures. 


44  SELECTION   IN    CLADOCERA    ON    THE    BASIS    OF 

given  in  the  seventh  vertical  column  of  the  tables  of  data  summarized 
by  two-month  periods  (tables  5  and  6). 

The  reproductive  indices  for  the  two  strains  are  plotted  in  figure 
Ic,  in  which  the  actual  values  are  represented  by  the  amount  of 
elevation  above  the  base-line  of  the  solid  and  open  circles.  The 
solid  circles  represent  the  reproductive  indices  for  the  plus  strain, 
the  open  circles  for  the  minus  strain.  The  distance  between  the 
solid  circle  and  the  open  circle  for  any  period  represents  the  difference 
in  reproductive  index,  the  solid  circle  or  the  open  circle  being  elevated 
the  more  above  the  base-line  as  the  plus  or  the  minus  strain  has  the 
larger  reproductive  index.  In  figure  2s  the  differences  between  the 
reproductive  indices  for  the  plus  and  minus  strains  are  shown,  a 
plus  advantage  being  shown  and  its  amount  indicated  by  the  eleva- 
tion of  the  solid  circle  above  the  base-line  and  a  minus  superiority  by 
the  amount  of  the  depression  of  the  open  circle  below  the  base-line. 

Throughout  the  first  of  the  three  periods  of  the  curve  for  Line 
695,  during  which  there  was  a  consistent  difference  between  the 
mean  reaction-times  of  the  two  strains  for  as  great  a  period  as  6 
months  (August  1912-January  1913),  the  plus  strain  had  the  lower 
reaction-time  and  was  slightly,  though  only  very  slightly,  the  more 
vigorous.  But  during  the  succeeding  four  months  (February-May 
1913;  see  figures  2s  and  2c)  the  plus  strain  had  a  considerably 
greater  advantage  over  the  minus  strain  in  vigor  and  yet  the  mean 
reaction-times  of  the  two  strains  were  very  near  together.  The 
fairly  wide  and  rather  consistent  difference  between  the  mean  re- 
action-times of  the  plus  and  minus  strains  for  6  months  during  1912- 
13  is  not  accounted  for  on  this  basis. 

For  the  ten-month  period  (August-September  1914  to  April- 
May  1915)  during  which  time  the  plus  strain  continuously  had  a 
higher  reaction-time  than  the  minus  strain,  the  plus  strain  was 
slightly  the  less  vigorous  of  the  two  during  6  of  the  10  months;  but 
during  the  other  4  months  the  reverse  was  true,  and  for  the  10  months 
as  a  whole  the  reproductive  vigor  of  the  two  strains  was  practically 
the  same.  For  the  seven-month  period  at  the  close  of  the  experi- 
ment, during  which  the  plus  strain  had  a  considerably  higher  re- 
action-time (was  less  reactive)  than  the  minus  strain,  the  plus  strain, 
judged  by  the  measures  applied  to  it,  was  actually  considerably  the 
more  vigorous  of  the  two  strains. 

Hence,  of  the  three  periods  of  the  curve  during  which  the  two 
strains  differed  consistently  in  reaction-time  for  as  much  as  6  months 
at  a  time,  only  one  of  these  differences,  that  for  the  second  period, 
and  only  a  portion  of  it,  could  be  accounted  for  on  the  basis  of  the 
more  vigorous  strain  having  been  the  more  reactive. 

Comparing  by  two-month  periods  for  the  entire  duration  of  the 
experiment,  one  finds  that  the  strain  of  Line  695  which  (on  the  basis 
of  its  reproductive  index)  was  the  more  vigorous  had  the  lower  re- 


A   PHYSIOLOGICAL   CHARACTER.  45 

action-time  during  14  of  these  periods,  while  in  12  two-month  periods 
the  more  vigorous  strain  had  the  higher  reaction-time.  For  one 
period  the  average  reaction-times  were  equal.  Compared  by  periods 
of  a  single  month,  the  strain  having  the  greater  vigor  had  a  lower 
reaction-time  in  23  cases  and  a  higher  reaction-time  in  26  cases. 
For  one  month  the  vigor  of  the  two  strains  measured  the  same,  for 
one  month  the  reaction-time  was  the  same,  and  for  three  months  the 
data  were  incomplete  and  a  comparison  could  not  be  made. 

Hence  in  the  selection  experiment  with  Line  695  no  relation 
is  discovered  between  fluctuations  in  general  vigor  and  mean  re- 
action-time, either  when  the  data  are  examined  month  by  month  or 
by  longer  periods  during  which  there  was  a  constant  difference  in 
reaction-time.  Whether  the  measure  of  vigor  is  taken  as  the  rate 
of  descent  or  as  the  reproductive  index,  any  reaction-time  differ- 
ences which  one  might  have  expected  to  find  explicable  on  the  basis 
of  differences  in  vigor  between  the  two  strains  quite  fail  to  appear  so. 

The  test  series  may  again  be  referred  to.  Three  of  the  four  of 
these  gave  differences  in  mean  reaction-time  which  are  statistically 
significant;1  for  one  the  difference  was  practically  nil.  Of  the  three 
differences  of  statistical  significance,  two  were  differences  in  which, 
in  opposition  to  selection,  the  plus  strain  had  the  higher  reaction- 
time  and  only  one  of  the  differences  was  in  the  direction  of  selection. 
Hence  their  net  effect  is  to  indicate  a  lack  of  effect  of  selection. 

Examined  as  a  whole  and  on  the  points  considered  in  detail,  it 
is  obvious  that  with  Line  695  there  was  no  effect  of  selection.  This 
is  by  no  means  surprising,  for  if  an  effect  of  selection  is  obtainable 
with  a  given  material  it  is  not  to  be  expected  that  such  effect  will 
necessarily  be  secured  every  time  it  is  attempted.  Selection  can- 
not be  effective  unless  germinal  variation  occurs  which  affects  the 
character  used  as  a  basis  for  selection;  and  there  seems  no  logical 
reason  for  supposing  that  germinal  variation  occurs  in  all,  or  to  the 
same  extent,  or  at  the  same  time,  in  all  material  which  is  otherwise 
apparently  entirely  similar. 

It  would  be  superfluous  to  make  such  an  extended  analysis  of 
the  data  for  each  of  the  lines  as  has  been  attempted  for  Line  695. 
Tables  of  data  and  diagrams  similar  to  those  for  Line  695  are  given 
for  all  the  lines  (though  some  of  these  lines  have  their  data  presented 
somewhat  less  fully),  but  it  has  not  seemed  necessary  to  make  the 
treatment  nearly  so  complete.  The  tabulated  summaries  and  the 
curves  plotting  reaction-times  and  the  reproductive  indices  render 
the  interpretation  rather  obvious  after  the  full  treatment  accorded 
Line  695.  Of  the  other  lines,  the  data  for  Line  757  alone  are  examined 
rather  extensively  because  of  the  interesting  result  within  that  line. 

1  The  times  at  which  these  test  series  were  conducted  are  indicated  by  the  roman  numerals 
on  figure  2c. 


46 


SELECTION    IN    CLADOCERA    ON    THE    BASIS    OF 


LINE  689. 

Line  689  was  obtained  from  the  surface-water  pond  in  an  open 
field  (Pond  I)  and  reared  in  the  laboratory  for  8  generations  before 
being  subjected  to  selection.  Selection  was  continued  16  months, 
44  generations  in  the  plus  strain  and  42  generations  in  the  minus 
strain,  when  the  experiment  was  terminated  by  the  accidental  loss 
of  the  plus  strain. 

The  data  are  presented  after  the  plan  followed  with  the  data 
for  Line  695  in  tables  9,  10,  11,  and  12,  and  in  figure  3,  A,  B,  and  c. 

For  the  first  4  months  of  selection  the  mean  reaction-time  for 
the  plus  strain  was  498.4  seconds  and  for  the  minus  strains  464.2 
seconds.  The  difference  was  +34.2  ±41.4  seconds  (table  11). 
For  the  year  (nearly),  from  August  1,  1912,  to  the  close  of  the  ex- 

TABLE  9. — Selection  data  summarized  by  two-month  periods  for  Line  689  plus. 


S, 

ts  i 

.3  >» 

03  — 

a 

o 

A 

A 

A 

, 

^ 

i 

% 

1  £ 

3 

M 

|| 

85 

"o 

C3 

1 

I 

c 

a 

3 

1 

1U 

"3  -T  J3 

I 

«2  "^ 

£ 

£ 

| 

£ 

"o 

03 

9 

S  « 

•*•  ^  g 

£ 

_03  "5 

g 

£ 

_£ 

^ 

c 

£ 

TJ 

3 

o 

*o 

•**  "o 

*°  *2  **- 

•3 

"ol  v_ 
3    O 

s 

3 
g 

i 

3 

9 

•8 

Time  period. 

"3 

JS 

«*H 

o 

§JI 

•I 

cS 

1 

"§ 

'5 

B 

§ 

I 

O 

- 

03 

fc-E  5P 

Mj 

T3    * 

i 

S 

i 

1 

•S 

03 

Generati 

6 
fc 

1 

Average 
brood. 

111 

|Jl 

.  '£ 

"*    03 

6  I 

Average 
time. 

Average 
time. 

y 
m 

C     « 

-'    •" 

Standard 

Probable 

Apr  .-May  1912  

10-14 

5 

28 

5.3 

11.1 

0.48 

3 

7 

244 

557 

14090 

503 

June-July  1912...  . 

15-19 

5 

26 

5.2 

9.0 

.58 

1 

5 

297 

664 

12825 

493 

Aug.-Sept.  1912... 

20-23 

6 

15 

2.3 

17.8 

.13 

0 

3 

315 

464 

5007 

334 

291  .U 

50:76 

Oct.-Nov.  1912.  .  . 

24-26 

a 

13 

5.5 

18.7 

.29 

1 

1 

226 

566 

4302 

331 

190.45 

35.63 

Dec.  1912-Jan.  1913 

27-33 

7 

60 

8  6 

9.0 

.96 

o 

4 

112 

531 

19043 

317 

Feb.-Mar.  1913.  .  . 

34-40 

8 

69 

8.3 

8  3 

1  00 

1 

3 

144 

476 

20838 

302 

Apr  .-May  1913.... 

41-47 

7 

60 

7.8 

7.9 

.99 

2 

0 

117 

436 

13468 

224 

June-July  1913.... 

48-52 

5 

42 

7.2 

8.2 

.88 

3 

1 

187 

688 

13775 

328 

22i!i2 

23^01 

TABLE  10. — Selection  data  summarized  by  two-month  periods  for  Line  689  minus. 


fc 

•*  i 

.2  >> 

' 

P 

. 

, 

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b 

. 

9y 

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1 

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11 

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i 

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S5 

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0 

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a 

£ 

JO'S 

£ 

£ 

.i 

i 

£ 

E 

TJ 

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g 

"8 

"0"° 

•8-g" 

>? 

3-0 

3 

3 
g 

i 

3 

*o 

L*      2 

"o 

0 
T3 

Time  period. 

Generations  of 

No.  of  broods. 

h 

0 

1 

1 

03       . 

O    O 

Average  No. 
per  first  broi 
average  age  o 

No.  of  negative 
dividuals. 

1-s 

15 

ii 

1 

fj 

83 

£ 

|| 

Sum  of  reactio 

'> 
1 
G    C 

S<;3 

Standard  devia 

Probable  error 

Difference  betv 
plus  and  mini 

u 

i 

03 
3 

Difference  divi 
ble  error. 

Apr-May  1012. 

10-13 

4 

9 

2.3 

13.3 

.17 

1 

2 

246 

586 

4048 

450 

+  53 

June-July  1912.  .  .  . 

14-17 

B 

20 

7.3 

9.0 

.81 

0 

3 

439 

728 

9415 

471 

+  22 

Aug.-Sept.  1912... 

18-19 

•2 

8 

4.0 

21.5 

.19 

0 

0 

138 

268 

1624 

203 

106.46 

25  39 

+  131 

56.70 

2.31 

Oct.-Nov.  1912.  .  . 

20-23 

6 

14 

1.3 

14.6 

.09 

0 

0 

155 

306 

3375 

241 

Hs.no 

26  79 

+   90 

44.57 

2.04 

Dec.  1912-Jan.  1913 

24-30 

7 

51 

7  3 

8  8 

.83 

2 

2 

114 

383 

13135 

QCQ 

-j-   59 

Feb.-Mar.  1913.  .  . 

31-36 

31 

5^2 

9.2 

0 

0 

17.S 

321 

8175 

2fvt 

+   38 

Apr.-  May  1913.... 

37-44 

8 

84 

9.1 

8.0 

l!l4 

1 

0 

113 

492 

21850 

260 

-  36 

June-  July  1913...  . 

45-50 

6 

35 

5.8 

7.5 

.77 

2 

0 

138 

468 

8722 

249 

130.89 

14.  '92 

+   79 

27.43 

2  .'88 

A   PHYSIOLOGICAL   CHARACTER. 


47 


TABLE  11. — Selection  summary  for  Line  689. 


Time  period. 


Apr.- July    31, 
1912 


Aug.    1,  1912- 
July  16,  1913 


Strain. 


fPlus.... 
[  Minus  . . 
fPlus.... 
[  Minus  . . 


9-19 

9-17 

20-52 

18-50 


T3  -£ 
.3  a> 


5.4 

8.2 
7.2 


6  g 
-.S 


54 

29 
259 


1, 

§1 

•sa 
I- 


-i 


12 


12 


271 

353 
174 


611 

665 
514 
S92 


498.4 
464.2 
295.1 
255. 1 


260.7 


194.8 
145.2 


23.9 

33.7 
8.2 
6.6 


s 


+34.2 


+40.0 


41.4 


10.5 


0.83 


3.71 


TABLE  12. — Same-day  broods.    Summary  of  data  for  Line  689. 


1 

4 

i 

t 

2 

fl     • 
S   ™ 

g 

. 

w 

s 

e 

^, 

>> 

OS 

>>  ^ 

"c«    OS 

3 

s 

§ 

•3 

g 

"«  "3 

3 

.§ 

.§ 

1 

^   § 

Time  period. 

Strain. 

1 

1 

11 

111 

.2  <u 

a.  a 

ll 

Ij 

3  a 

a)  'o 

»£ 

o  2 

fl^ 

.2         Q        flj 

SB*  rl 

Si    CJ 

fl 

0)    M 

•3 

0 

fc 

i  & 

J<<  P« 

o 

0 

P 

IK 

jl 

" 

C   0 
11 

S" 

to  S 

S  a 

Apr.     9,  1912-July   31, 

f  Plus  .  . 

1 

5.0 

5 

0 

3 

315 

900 

712 

+  163 

1912  

(  Minus 

1 

4.0 

4 

0 

0 

^^0 

690 

549 

AUR.     1,  1912-July  31, 

{  Plus  .  . 

J 

10 

8.9 

89 

4 

2 

125 

464 

295 

+  3 

1913  

.  _„ 

ago 

periment  with  this  line  in  July  1913,  the  mean  for  the  plus  strain 
was  295.1  seconds  and  for  the  minus  255.1  seconds.  The  difference 
was  -f-40.0rtl0.5  seconds,  3.71  times  its  probable  error.  There 
was  only  one  same-day  brood1  for  the  four-month  period,  but  there 
were  ten  same-day  broods  during  the  later  year-period.  For  these 
same-day  broods  likewise  the  reaction-time  for  the  minus  strain  was 
lower  than  for  the  plus  strain,  but  the  difference  was  only  3  seconds 
(table  12). 

It  is  an  interesting  fact  that  in  this  line,  in  opposition  to  selec- 
tion, the  minus  strain  persistently  (except  for  1  two-month  period, 
see  table  10)  maintained  a  lower  reaction- time  than  the  plus  strain. 
This  difference,  while  not  large  for  the  greater  part,  is  so  general 
that  it  can  scarcely  seem  to  be  merely  coincidental.  The  same-day 
broods  (table  12)  show  the  minus  broods  with  a  smaller  reaction- 
time,  but  for  the  only  significant  group  of  these  the  difference  is  so 
slight  (3  seconds)  as  scarcely  to  be  suggestive.  

1For  this  pair  of  very  small  broods  the  mean  of  the  plus  strain  was  163  seconds   greater 
than  that  for  the  minus  strain. 


48 


SELECTION   IN    CLADOCERA   ON   THE    BASIS   OF 


1.00 


50 


00 


4-5         8-9 
1912 


12-1 


4-5 


1913 


- 

I 

T 

B 

T 

1 

T" 

.50 
.25 
OO 
.25 

\ 

\ 

r 

i          J 

There  were  more  over-time  individuals  in  the  plus  strain,  24  as 
compared  with  7  in  the  minus  strain.  The  average  minimum  and 
maximum  reaction-times  likewise  indicate  greater  reactiveness  in 
the  minus  strain  for  the  last  12  of  the  total  16  months  of  the  experi- 
ment. The  average  minimum  reaction-time  for  all  the  broods  tested 
in  the  plus  strain  for  this  last  year  was  174  seconds  (table  11),  while 
the  corresponding  average  for  the  minus  strain  was  136  seconds, 
indicating  that  the  most  reactive  individuals  in  the  minus  strain  had 
a  lower  reaction-time  by  38  seconds 
than  the  corresponding  individuals 
in  the  plus  strain.  A  corresponding 
comparison  for  the  maximum  re- 
action-times for  the  two  strains 
gives  514  seconds  as  the  average 
maximum  for  the  plus  and  392 
seconds  as  the  average  for  the  minus 
strain,  the  difference  (122  seconds) 
indicating  that  the  individuals 
with  the  highest  reaction-times  in 
the  plus  strain  were  31  per  cent 
slower  in  reacting  than  the  corre- 
sponding individuals  in  the  minus 
strains.  These  portions  of  data 
tend  to  confirm  a  greater  re- 
activeness for  the  minus  strain 
than  for  the  plus  strain. 

Comparison  of  the  curves  for 
the  two  strains,  of  Line  689  may 
be  made  with  the  combined  mean 
reaction-time  curves  for  all  the 
plus  and  for  all  the  minus  strains 
of  those  Daphnia  pulex  lines  be- 
tween the  two  strains  of  which 
there  were  no  consistent  differences 
in  reactiveness — Lines  691,  695, 

m'JiA        «^1    TCI    /~~  ^    C.    ,      ^    o^A      signmcant    reaction-time    ainerences  cue 
,  714,  and  751  (see  figure  3c).  arise_ Lines  691f  695>  713f  714|  and  751< 

Inspection  of  these  curves  shows 
that  for  3  two-month  periods  (August  1912-January  1913)  the 
plus  strain  of  Line  689  was  abnormally  high,  while  the  minus 
strain  of  Line  689  was  abnormally  low  in  reaction-time,  and  that 
aside  from  these  3  two-month  periods  the  two  strains  of  Line  689 
did  not  differ  so  greatly  in  their  reactiveness.  It  would  seem  possible 
that  the  pronounced  divergence  in  reactiveness  between  the  two 
strains  of  this  line  for  the  period  indicated  may  have  been  due 
to  the  coincidence  of  unusual  reactiveness  of  the  minus  strain  and 
slight  reactiveness  of  the  plus  strain  brought  about  by  differential 


4-50 


300 


150 


00 


4-5        8-9 
1912 


12-1        4-5 


1913 


FIGURE  3. — Line  689. 

A.  Reproductive  indices,  actual  values. 

B.  Reproductive  indices,  superiority. 

C.  Reaction-time    curves    with    superim- 
posed curves  (in  faint  lines)  representing  the 
combined  reaction-time  means  for  the  plus  and 
minus  strains  of  all  Da phnia  pulex  lines  in  which 
significant    reaction-time    differences  did  not 


A   PHYSIOLOGICAL   CHARACTER. 


49 


environmental  factors  (see  page  140).  However,  in  4  of  the  5  other 
two-month  periods  of  the  experiment,  the  minus  strain  was  the  more 
reactive  (the  combined  mean  was  42  seconds  greater  for  the  plus 
strain),  and  it  is  possible  that  there  really  existed  a  genetic  differ- 
ence between  the  two  strains  of  this  line. 

Perhaps  the  most  that  can  be  said  "regarding  Line  689  is  that 
the  persistently  lower  reaction-time  in  the  minus  strain  is  suggestive 
of  a  genetic  difference  in  the  reverse  of  selection.  It  is  much  to  be 
regretted  that  this  line  could  not  have  been  continued  longer  or  that 
large  test  series  were  not  conducted  in  order  to  secure  better  evidence 
concerning  the  real  persistence  of  this  apparent  difference  in  reaction- 
time. 

With  regard  to  any  relation  between  mean  reproductive  vigor 
and  mean  reaction-time  in  Line  689,  it  may  be  said  that  no  relation  is 
discoverable.  On  the  whole,  the  plus  strain  was  somewhat  the  more 
vigorous  (see  figure  3,  A  and  B)  while  the  minus  strain  was  the  more 
reactive.  Further,  there  is  an  entire  lack  of  coincidence  of  lowered 
reaction-time  means  with  higher  reproductive  indices. 

The  influence  of  general  environmental  factors  is  seen  in  the 
somewhat  coincident  up-and-down  movements  of  the  curves  for  the 
two  strains. 

LINE  691. 

The  data  for  Line  691  are  presented  in  tables  13  and  14  and  in 
figure  4.  This  is  one  of  the  original  lines  of  D.  pulex.  Selection  was 
begun  in  the  seventh  generation  and  continued  for  87  and  89  genera- 
tions in  the  two  strains.  The  experiment  was  terminated  by  the 
loss  of  the  plus  strain  after  27  months  of  selection. 

For  the  first  4  months  of  the  experiment  the  mean  reaction- 
times  for  the  plus  and  minus  strains  (only  40  and  35  individuals 
respectively)  were  480.9  and  434.9  seconds  (table  13).  The  plus 

TABLE  13. — Selection  summary  for  Line  691. 


Time  period. 


Apr.  12,  1912- 
July  31,  1912 

Aug.  1,  1912- 
July  31,  1913 

Aug.  1,  1913- 
June  12,  1914 


Strain. 


Plus.. 
Minus 

Minus 
Plus.  . 
Minus 


10-19 
11-18 
20-55 
19-57 
56-93 
58-95 


11 

S   « 
d'S 

flj    O 


3.7 
5.0 
5.9 
6.4 
7.3 
7.0 


i: 


40 

35 

225 

257 
285 
273 


•32 


•s 


10 


347 
169 
142 
/#> 
171 
**« 


743 

577 
373 
459 
680 


480.9 
434.9 
248.8 

391  !o 


299.0 
300.5 
164.5 
220.6 
265.8 
JBfiS.jf 


31.9 

34.3 
7.4 

10.6 
10.9 


Q  * 


+46.0 
-47  '.8 


46.8 


It. 9 

15.i 


0.98 


50 


SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 


TABLE  14. — Same-day  broods.    Summary  of  data  for  Line  691. 


l_ 

1 

3 

^ 

c 

c 

a 

S 

« 

i 

0 

0 

W) 

c 

c  •* 

"S 

1 

i 

S 

S    . 

§ 

G 

i 

u 

'"%  3 

i 

i 

i 

•s 

§ 

2  S 

«§ 

Time  period. 

Strain. 

I 

0 

6 

ndividuals. 

"3 

individuals  f 
an  end  of  the 

B  minimum 

B  maximum 

individual 

rd  deviation 

le  error  of  me 

ice  between  r 
ad  minus  strj 

T3 

"S 
j 

ace  divided  b 
or. 

"o 

fl 

*o 

*l 

15  -g 

«  g 

B 

M 

a  a 

f 

A 

c3 

|    B 

s  n 

fe  « 

£ 

^^ 

o 

Is3 

IS 

>-J 

!* 

«| 

i 

I 

S"5 

£ 

0s 

Apr     8   1912 

(  Plus 

3  5 

7 

o 

3 

285 

900 

634 

+  331 

July  31    1912 

0 

g 

0 

j 

125 

SOS 

Aug     1    1912 

i    Plus 

<} 

3  5 

28 

1 

1 

169 

260 

192 

98  69 

12  58 

July  31,  1913 

(  Minus  .  . 

0 

/>.,-? 

« 

ff 

4tf/ 

S4S 

272.26 

28.  S  4 

-156 

S/.00 

5.03 

Aug      1    1913 

i    Plus 

6  7 

40 

g 

7 

168 

781 

413 

+  43 

June   9   1914 

{  Minus 

^> 

7  S 

7 

5 

JO2 

S./3 

370 

mean  was  larger  by  46  ±46.8  seconds.1  For  the  year,  August  1912- 
July  1913,  the  two  means  were  248.8  and  296.6  seconds  (225  and  257 
individuals).  The  difference,  -47. 8 ±11. 9  seconds,  was  4.02  times 
the  probable  error.  The  same-day  broods  for  this  year-period,  of 
which  there  were  8,  gave  a  mean  of  192  seconds  for  the  plus  and  of 
348  seconds  for  the  minus  strain  and  a  difference  of  —156  ±31. 00 
seconds — 5.03  times  the  probable  error  (table  14).  In  spite  of  the 
irregular  course  of  the  curves,  the  data  for  this  year  might  seem  to 
indicate  a  possible  (though  slight)  effect  of  selection,  except  for  the 
fact  that  the  further  history  of  the  line  does  not  bear  out  this  inter- 
pretation. 

For  the  following  year  (nearly),  which  concludes  the  data  for 
this  line,  the  mean  for  the  plus  strain  was  391.0  seconds  (285  indi- 
viduals); for  the  minus  strain  416.2  seconds  (273  individuals).  The 
difference  was  -25. 2 ±15.2  seconds  (table  13)  and  is  not  of  statisti- 
cal significance.  There  were  6  same-day  broods  for  this  period,  for 
which  the  plus  mean  was  413  seconds  and  the  minus  mean  370 
seconds  (table  14).  This  difference  (-1-43  seconds),  while  not  of 
statistical  significance,  is  in  the  reverse  of  an  effect  of  selection.  No 
test  series  were  obtained  for  Line  691. 

Figure  4  shows  the  curves  of  the  mean  reaction-times  for  the 
two  strains  for  Line  691  by  two-month  periods.  The  curves  are 
quite  irregular  and  indicate  considerable  fluctuation  in  mean  re- 
action-times, particularly  for  the  minus  strain,  which,  except  for  its 
wide  fluctuations,  was  persistently  the  less  reactive  of  the  two  strains 
after  the  first  4  months  of  selection.  The  curves  for  the  two  strains 
follow  each  other  in  a  general  way,  but  are  so  irregular  that  they 
cross  8  times.  The  rather  persistently  (though  only  slightly)  higher 
reaction-time  of  the  minus  strain  is  suggestive  of  a  possible  slight 

1  There  were  only  2  same-day  broods,  involving  totals  of  only  7  individuals  in  the  plus  and 
8  individuals  in  the  minus  strain.  The  means  were  634  and  303  seconds,  the  difference  being 
+331  seconds.  These  broods  came  in  July,  when  the  plus  strain  was  relatively  slightly  reactive 


A   PHYSIOLOGICAL   CHARACTER. 


51 


effect  of  selection  within  this  line.  However,  the  actually  smaller 
difference  in  mean  reaction-times  during  the  last  year  as  compared 
with  the  preceding  year  does  not  lend  support  to  this  supposition, 
and  the  same-day-brood  data  for  the  last  longer  period  are  in  oppo- 
sition to  an  effect  of  selection,  so  that  a  result  of  selection  within 
Line  691  must  be  considered  very  questionable. 

Obtaining  the  data  for  reproductive  vigor  (from  the  daily 
laboratory  notes)  is  laborious  and  was  not  done  for  Line  691  and  a 
number  of  the  other  lines  in  which  there  was  obviously  no  effect  of 
selection  or  in  which  the  results  were  inconclusive. 

The  parallel  effect  of  broad  environmental  influences  upon  the 
reaction-time  means  for  the  two  strains  is  shown  in  the  curves 
(figure  4)  for  the  two  strains,  they  in  a  general  way  following  the 
same  course. 


600 


450 


300 


150 


00 


V5          8-9 
1912 


12-1 


8-9 


12-1 


1913 


1914 


FIGURE  4. — Line  691.    Reaction-time  curves. 


LINE  711. 
Tables  15, 16, 17,  and  18  and  figure  5  present  the  data  for  Line711, 

TABLE  15. — Selection  data  summarized  by  two-month  periods  for  Line  711  plus. 


a 

2 

i 

a 

a 

d 

1 

1 

•N 

| 

1 

1 

a 

•g 

1 

| 

I 

£ 

2 

| 

i 

•s 

a* 

8 
« 

M 

a 

IH 

^5 

Q 

1 

1 

d 

a 

o 

j>> 

3  *O 

3 

g 

a 

'43 

•a 

Time  period. 

•s 

co 

I 

ITS 

.5 

•p 

.2 

b 

TO 

1 

1 

6 

}| 

la 

1 

o 

03 

1 

I 

1 
•2 

0) 

•3 

2 

ol 

*Q  -3 

?« 

?« 

"o 

&     1) 

3 

2 

i 

d 

9 

o 

<i  I 

p.1 

p.i 

a 

2| 

a 

i 

"o 

o 

z 

H 

55  "* 

55  * 

^^ 

<j^ 

03 

^^ 

OQ 

£ 

Apr.-May  1912.... 

7-12 

6 

30 

3 

8 

299 

783 

13605 

454 

308.91 

38.040 

June-July  1912.... 
Aug.-Sept.  1912...  . 
Oct.-Nov.  1912...     . 

13-17 
18-19 
20-23 

6 
2 

4 

22 

6 
14 

0 
0 
2 

10 
0 

o 

345 
144 
124 

668 
220 
449 

11805 
1043 
3662 

537 
174 
262 

41.77 

"ll.S.. 

Dec.l912-Jan.1913.. 

24-31 

8 

83 

1 

4 

114 

506 

22875 

276 

Feb.-Mar.  1913...    . 

32-36 

5 

34 

0 

0 

141 

397 

9225 

271 

Apr.-May  1913  

37-43 

7 

62 

1 

0 

119 

349 

13095 

211 

June-July  1913  

44-50 

7 

36 

1 

0 

148 

343 

6792 

189 

96.75 

'io'.88 

52  SELECTION    IN    CLADOCERA    ON   THE    BASIS    OF 

TABLE  16. — Selection  data  summarized  by  two-month  periods  for  Line  711  minus. 


! 

cting  in- 

o 
fl 

1 

1 

00 

| 

i 

i 

8 

S  . 

2  § 

erence. 

i 

S 

Time  period. 

of  descei 

young  t 

I 

j*S 

ll 

nimum  i 

ximum  i 

ion-time 

*o 

o 

1 

a 

0 

a  s 

g  "w 

li 

1 

i 

§ 

g 

0 

Ija 

T3   "" 

a 

a 

i 

1 

T3 

I 

*  a 

« 

T3 

^ 

0 

fc 

s| 

"•   oJ 

§ 

£ 

"8 

J2 

fi 

JD 

g 

c3 

0 

~? 

<0^ 

a1,* 

2  « 

° 

s  s 

"3 

03 

03 

8* 

O 

0 

1 

& 

|£ 

1* 

02 

g  fa 

3* 

I 

1 

s* 

£ 

M 

Apr-May   1912... 

7-12 

6 

25 

1 

14 

458 

900 

17440 

698 

265.71 

35.84 

-244 

52.27 

4.66 

June-July   1912.  .  . 

13-18 

fi 

?A 

1 

8 

308 

643 

11381 

474 

+63 

Aug.-Sept.  1912.  . 

18-19 

3 

5 

2 

0 

348 

572 

2385 

477 

171.16 

51.63 

-303 

52.89 

5.33 

Oct.-Nov.  1912.  . 

20-25 

7 

17 

3 

0 

229 

335 

5019 

295 

-33 

Dec.  1912-Jan.  1913 

26-31 

6 

41 

1 

113 

380 

12484 

304 

—28 

Feb.-Mar.  1913.. 

32-38 

7 

43 

2 

ni 

418 

12550 

W? 

—21 

Apr.-May   1913.. 

39-45 

7 

40 

1 

n 

131 

341 

9515 

238 

—27 

June-July   1913.. 

46-51 

6 

44 

10 

2 

132 

573 

12275 

279 

201.87 

20.53 

-90 

23.23 

3.87 

Time  period. 


Apr.    1912- 
July  31,  1912 


Aug.    1,  1912- 
July  31,  1913 


Strain. 


TABLE  17. — Selection  summary  for  Line  711. 


f  Plus.... 
1  Minus  . . 
fPlus.... 
t  Minus  . . 


6-17 

6-18 

18-50 

18-61 


12 


4.3 


7.1 
5.3 


II 

I  s 

I1 

.S  g> 


52 


235 

190 


»4 


s^ 


l-s 


18 


322 
376 
129 


727 
760 
397 
419 


8.8 


488.7 


241.2 


327.7 
3/5.  / 
163.3 
193.8 


30.7 

30.4 

7.2 

9.5 


-59.5 


-45.7 


43.1 


11.9 


3 


2 


t.31 


3.54 


TABLE  18. — Same-day  broods.    Summary  of  data  for  Line  7 1 1 . 


9 

i    . 

•38 

i 

a 

3 

S 

Sg 

° 

J 

•&1 

S'j'S 

a 

3 

a 

•rj 

^ 

. 

"o 

T3 

a  I  41 

i 

"  S 

1 

^2  9 

V 

'? 

'•*3   > 

fl     *^     03 

•g  o5 

a  a 

•8 

Time  period. 

Strain. 

1 

£"i 

1 

§"§ 

"3^ 

11 

•^ 

03     ^ 

1  g 

si 

"3 

if- 

*O 

o| 

•s^s 

s$ 

'a  "^ 

li 

* 

3* 

^ 

1- 

|jsa 

^s 

^" 

1^ 

5  s 

Mar.  26,  1912-July  31, 

(Plus.. 

3 

4.0 

12 

0 

7 

477 

900 

680 

+  42 

1912  

1  *r- 

AUR.     1,  1912-July   31, 

(  Minus 
(Plus.. 

3 
6 

3.7 
8.3 

11 
50 

/ 
1 

6 
6 

353 
128 

820 
398 

638 
224 

1913 

.  Minus 

6 

5.5 

33 

6 

0 

132 

348 

£31 

—  7 

A   PHYSIOLOGICAL   CHARACTER. 


53 


75O 


600 


450 


300 


This  is  one  of  the  original  lines  of  D.  pulex  obtained  from  Pond 
II,  the  spring-fed  pond  in  the  woods,  during  November  1911.  Se- 
lection was  begun  in  the  sixth  laboratory  generation  and  continued 
for  15  months,  for  45  generations  in  the  plus  and  46  generations  in 
the  minus  strains. 

For  the  first  4  months  of  the  experiment  with  this  line  the 
means  were,  for  the  plus  strain  488.7  seconds  (52  individuals)  and 
for  the  minus  strain  588.2  seconds  (49  individuals).  The  difference 
(-99.5 ±43.1  seconds)  was  2.31  times  the  probable  error  (table  17). 
The  mean  reaction-time  for  the  plus  strain  for  the  first  two-month 
period  was  244  seconds  less  than  that  for  the  minus  strain,  but  for 

the  second  two-month  period  the 
minus  strain  had  the  lower  reaction- 
time  by  63  seconds  (tables  15  and 
16).  There  were  3  same-day  broods, 
for  which  those  of  the  plus  strain  had 
a  higher  mean  reaction-time  by  42 
seconds  (table  18) .  The  data  for  this 
first  4  months  of  selection  are  not  as 
satisfactory  as  that  obtained  later 
and  wide  fluctuations  are  less  sur- 
prising. 

For  the  remaining  year  (nearly) 
of  the  experiment  with  this  line,  the 
mean  for  the  plus  strain  was  241.2 
seconds  (235  individuals) ;  that  for  the 
minus  strain  was  286.9  seconds  (190 
individuals).  The  same-day  broods, 
of  which  there  were  6,  have  means 
differing  by  only  —7  seconds.  Exam- 
ination of  the  data  (tables  15  and  16) 
and  the  curves  (figure  5)  for  this  line 
shows  consistent  differences  in  mean 

reaction-times  during  this  year-period.  The  wide  divergence  in  mean 
reaction-times  for  August— September  is  a  chance  result  due  to  there 
being  extremely  few  tested  individuals  during  this  period.  But  for 
the  following  5  two-month  periods  the  numbers  of  tested  individuals 
were  greater  and  the  rather  uniform  courses  of  the  curves  are  sugges- 
tive of  a  difference  due  to  selection. 

Comparison  of  the  curves  for  the  means  of  the  two  strains  of 
this  line  with  the  combined  mean  reaction-time  curves  for  Lines 
691,  695,  713,  714,  and  751  (in  which  genetic  differences  presumably 
did  not  arise)  shows  (figure  5)  that  the  plus  strain  of  Line  711  after 
the  first  4  months  of  the  experiment  was  somewhat  more  reactive 
than  the  other  plus  strains.  The  means  for  the  minus  strain  in 
general  were  higher  than  the  combined  means  for  the  other  minus 


00 


8-9 


1912 


1913 


FIGURE  5. — Line  711. 
Reaction-time  curves  with  superim- 
posed curves  representing  the  combined 
reaction-time  means  for  the  plus  and 
minus  strains  of  all  Daphnia  pulex  lines 
in  which  significant  reaction-time  differ- 
ences did  not  arise. 


54 


SELECTION    IN    CLADOCERA    ON   THE    BASIS    OF 


strains.    Hence  this  comparison  affords  further  evidence  of  an  effect 
of  selection  in  Line  711. 

An  effect  of  selection  within  Line  711  is  not  certainly  established, 
however,  in  view  of  the  lack  of  sufficient  confirmation  from  the  same- 
day-brood  data.  But  these  data  were  not  very  numerous  and  it  is 

TABLE  19. — Selection  summary  for  Line  713. 


•S 

A 

Cj 

0 

a 

a 

L 

d 

i 

0) 

, 

t 

3  T5 

II 

TJ  -2 

|| 

1 

iN 

reactio 

reactio 

1 

•3 

a* 

0] 
0) 

B 
2 

1 

ta 

a 

Time  period. 

Strain. 

0 

1 

~ 

*O  ^ 

do 

55  "g 

1| 

negatively  rei 
nals. 

individuals  : 
an  end  of  th< 

je  minimum 

;e  maximum 

individual 

ird  deviation 

a 

0 

.2 

Is 

•3 

nee  divided  b 
ror. 

I 

"o 

d-° 

—  .S 

°| 

0  "w 

c?  -' 

a  <o 

i 

I 

£  o 

"S 

2  S 

O 

^6 

I* 

g* 

6  | 

P 

P 

2  S 

3* 

1 

|1 

1 

§3 

Apr.    1912- 
July  31,  1912 

f  Plus.... 
I  Minus.. 

9-18 
10-19 

10 
12 

5.4 
5.7 

54 

3 

2 

15 
14 

232 

644 

743 

476.7 
459.7 

314.5 

277.4 

28.9 
24.0 

+  17.0 

37.5 

.45 

Aug.    1,  1912- 

f  Plus  .... 

19-53 

37 

6.4 

236 

3 

5 

139 

401 

244.6 

163.0 

7.2 

July  31,  1913 

I  Minus  .  . 

20-54 

36 

5.0 

203 

7 

7  S3 

437 

255.4 

755.5 

8.9 

'-40.8 

77^5 

3  55 

Aug.    1,  1913- 

(  Plus.... 

54-96 

42 

9.1 

383 

28 

72 

202 

667 

431.3 

287.7 

9.9 

+  27.0 

13.9 

l!94 

July  31,  1914 

I  Minus  .  . 

66-93 

40 

5.2 

327 

20 

774 

732 

40^.3 

261.0 

9.7 

Test  series. 

(  Plus.... 

80 

15 

17.9 

268 

31 

12 

113 

708 

265.9 

217.1 

8.9 

+  18  6 

ii.e 

1.58 

Mar.  1914... 

I  Minus  . 

77 

15 

75.2 

273 

720 

629 

247.3 

757.0 

7.4 

Feb.     1914- 

f  Plus.  . 

76-84 

9 

8.3 

75 

4 

15 

149 

758 

425.3 

Apr.  31.  1914 

I  Minus  . 

73-53 

10 

77 

9 

245 

722 

457    0 

—32.3 

Aug.     1,  1914- 

(  Plus... 

97-136 

40 

ll!9 

477 

43 

39 

145 

633 

306.0 

241.3 

7.5 

July    7,  1915 

I  Minus  . 

94-139 

46 

72.0 

550 

55 

147 

698 

349.6 

236.9 

0.5 

-43  '.6 

10.  l' 

4  '.32 

Test  series, 

(  Plus.  .. 

125 

9 

20.0 

180 

31 

19 

91 

778 

342.9 

247.2 

12.4 

Mar.  1915... 

I  Minus  . 

725 

9 

73.5 

775 

77 

104 

756 

354-4 

207.7 

73.2 

-41.6 

18.2 

2^25 

Test  series, 

{  Plus... 

126 

32 

39.6 

1266 

66 

161 

96 

817 

372.7 

265.2 

5.0 

+   1.0 

6.9 

.14 

Mar.  1915.  .  . 

(  Minus  . 

720 

SJ 

33.0 

7205 

247 

If  / 

95 

507 

377.7 

245.4 

4.7 

Feb.     1,  1915- 

(  Plus  .  .  . 

120-130 

11 

14.3 

157 

19 

17 

104 

678 

335.8 

+  19.3 

Apr.  30,  1915 

I  Minus  . 

119-130 

12 

77.5 

735 

14 

10 

102 

002 

370.5 

TABLE  20. — Same-day  broods.    Summary  of  data  for  Line  713. 


K 

c 

$ 

ft 

i 

A 

d 

^ 

. 

03 

M 

! 

M-* 

c  c 

'•3 

••£ 

*» 

S 

S    . 

g 

3 
O 
>> 

1: 

£ 

2 

§ 

0 

1 

2  S 
S'3 

S3 

>, 

J2 

Time  period. 

Strain. 

d 

mber  of 

viduals. 

>i 
1 

a* 

*3  ««-« 

3   0 

|1 

linimum 

S 
^ 
S 
•3 

OJ 

ividual 

c 

•s 

between 
ninus  stl 

rror  of  d 

divided 

0 

6 

Z 

Average  nu 
brood. 

I 

0 

6 
fc 

No.  of  negj 
dividuals. 

•S  w 

.£§ 

°1 
°» 

Average  rr 
time. 

Average  rr 
time. 

j 

a  a> 
S| 

Standard  c 

0) 

« 

Difference 
plus  and  i 

Probable  e 

Difference 
ble  error. 

Apr.  13,  1912- 

(  Plus 

g 

2  7 

8 

2 

2 

338 

403 

490 

+53 

July  31,  1912 

I  Minus  .  . 

9 

ff  0 

9 

0 

1 

325 

660 

437 

Aug.     1,  1912- 

(  Plus.... 

14 

7.1 

100 

1 

1 

130 

389 

234 

154.42 

i6.42 

July  31,  1913 

t  Minus  .  . 

M 

74 

/03 

S 

.9 

948 

#41 

322 

909.  (M 

73.53 

-55 

17.31 

5.05 

Aug.     1,  1913- 

{  Plus.... 

7 

8.9 

62 

5 

24 

343 

829 

620 

290.23 

24.86 

+  240 

34.44 

6.96 

July  31    1914 

7 

5  4 

69 

5 

7 

75^ 

764 

350 

'?/  4s 

23  54 

Aug.     1,  1914- 
July    7,  1915 

(  Plus  .  . 
I  Minus 

14 

^  / 

13.8 
13  / 

193 
154 

15 

29 

17 
IS 

125 

146 

690 
655 

305 
333 

—25 

difficult  to  ignore  the  very  consistent  differences  between  the  two 
strains  after  the  first  4  months  of  the  experiment.  It  is  to  be  re- 
gretted that  no  test  series  was  conducted  with  this  line. 

This  case  is  very  similar  to  that  for  Line  689,  with  which  there 
was  a  similar  difference,  but  in  the  reverse  direction. 


A   PHYSIOLOGICAL   CHARACTER. 


55 


With  Line  711,  as  well  as  with  most  of  the  other  lines,  the  courses 
of  the  curves  for  the  two  strains  illustrate  the  marked  effect  of 
environmental  influences  upon  reaction-time  (figure  5). 

LINE  713. 

The  data  for  Line  713  are  shown  in  tables  19  and  20  and  in 
figure  6.  Scarcely  more  than  a  glance  at  figure  6  is  necessary  to 
convince  one  that  with  Line  713  there  was  no  effect  of  selection, 
though  the  selection  data  for  the  last  year  period  are  suggestive  of  a 
possible  selective  effect.  Environmental  influences  were  obviously 
at  play  in  directing  the  general  course  of  the  curves,  which  follow 
each  other  in  an  interesting  way.  The  curve  for  the  plus  strain  is 
quite  irregular,  while  that  for  the  minus  strain  is  surprisingly  free 
from  large  minor  fluctuations. 


600 


450 


300 


150  - 


00 


4-5  8-9  12-1  A-5  8-9  12-1  4-5  8-9  12-1  A--5 

1912  1913  ISM-  1915 


FIGURE  6. — Line  713.     Reaction-time  curves. 


For  the  first  4  months  the  means  for  the  two  strains  were 
476.7  and  459.7  seconds  (table  19).  The  difference  is  +17.0 ±37.5 
seconds.  There  were  3  very  small  same-day  broods  for  each  strain, 
with  the  average  for  the  plus  strain  53  seconds  the  larger  (table  18). 
The  large  upshoot  in  the  curve  for  the  plus  strain  for  June— July  is 
from  an  average  obtained  from  a  very  small  number  of  individuals. 

The  data  for  the  year,  August  1912-July  1913,  show  these 
means  for  the  plus  and  minus  strains:  244.6  and  285.4  seconds.  The 
difference  is  —40. 8  ±11. 5  seconds,  3.55  times  the  probable  error. 
The  14  same-day  broods  for  this  period  have  as  averages  233.7  and 
321.9  seconds,  the  minus-strain  average  being  larger  by  88  ±17.31 
seconds.  The  extremely  fluctuating  course  of  the  curves  for  this 
year-period  indicates  that  the  differences  obtained  are  not  due  to 
an  effect  of  selection. 

During  the  next  year,  August  1913-July  1914,  the  plus  mean 
is  higher  by  27.0  ±13. 89  seconds.  (The  plus  mean  is  431.3  seconds 
and  the  minus  mean  404.3  seconds.)  The  7  same-day  broods  averaged 


56        SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

620  and  380  seconds,  the  plus  mean  being  240  ±34.44  seconds  the 
larger.1 

A  small  test  series  (268  and  273  individuals  in  the  two  strains) 
conducted  during  March  1914,  gave  265.9  and  247.3  seconds  as  the 
plus  and  minus  mean  reaction-times  (table  19).  The  difference 
(  +  18. 6  ±11. 62  seconds)  was  1.58  times  the  probable  error. 

For  the  final  year  of  the  experiment  with  Line  713  (August 
1914-July  7,  1915)  the  means  were  306.0  and  349.6  seconds.  The 
difference  was  —43. 6 ±10.1  seconds,  or  4.32  times  the  probable 
error.  For  the  14  same-day  broods  the  means  were  305  and  333 
seconds,  the  difference  being  only  —  28  seconds.  Two  test  series  were 
conducted  during  March  of  this  year.  The  first  consisted  of  a  com- 
paratively small  number  of  individuals  (180  and  178).  The  means 
were  342.9  and  384.4  seconds.  The  difference  (-41.5dbl8.15 
seconds)  is  2.28  times  the  probable  error  and  is  not  a  significant 
difference.  A  much  larger  and  more  adequate  test  series  was  con- 
ducted after  that  just  referred  to.  The  numbers  were  1,266  and 
1,268  for  the  plus  and  minus  strains,  respectively.  The  means  were 
372.7  and  371.7  seconds.  The  difference  was  only  +1.0  ±6.88 
seconds.  This  test  series  was  conducted  with  every  care  and  it  is 
believed  may  safely  be  regarded  as  a  crucial  test.  It  brings  out 
clearly  enough  that,  in  spite  of  the  small  but  rather  consistent  dif- 
ference in  mean  reaction-times  during  the  last  year  of  the  experi- 
ment, there  is  not  an  effect  of  selection  in  Line  713. 

LINE  714. 

Line  714  was  one  of  the  original  lines  of  D.  pulex.  Selection 
was  begun  in  the  eighth  generation  and  continued  for  more  than  53 
months,  184  generations  in  the  plus  strain  and  196  generations  in 
the  minus  strain.  The  data  for  Line  714  are  given  in  tables  21  and 
22  and  in  figure  7. 

For  the  first  4  months  of  the  experiment  the  mean  reaction-time 
for  the  plus  strain  was  495.4  and  for  the  minus  strain  618.9  seconds 
(table  21).  The  difference  was  rather  large  (-123.5 ±32.0  seconds, 
3.86  times  the  probable  error).  There  were  only  two  small  same- 
day  broods  for  this  period  (table  22),  for  which  the  mean  for  the 
minus  strain  was  231  seconds  the  larger. 

For  the  year-period  (August  1,  1912-July  31,  1913)  the  mean 
reaction-time  for  the  plus  strain  was  309.2  seconds  and  that  for 
the  minus  strain  278.5  seconds.  The  difference  was  +30. 7  ±12.4 
seconds  (2.48  times  the  probable  error).  This  difference  is  in  the 
reverse  direction  to  that  for  the  earlier  (four-month)  period.  The  3 
same-day  broods  had  a  mean  for  the  plus  strain  226  ±27. 62  seconds 
the  larger. 

lOi  these  7  same-day  broods,   4  occurred   during  May,  when  the  plus  strain  for  some  un- 
accounted-for reason  was  rather  non-reactive  for  D.  pulex. 


A   PHYSIOLOGICAL   CHARACTER. 


57 


For  the  next  year-period  (August  1,  1913-July  31,  1914)  the 
means  were  406.4  and  439.2  seconds.     The  difference  (-32.8  ±14.5 

TABLE  21. — Selection  summary  for  Line  714- 


J 

j, 

i 

o 

a 

1 

, 

a 

jb 

S" 

7 

ll 

>    <n 

il 

's 

|| 

1 

oj 

i 

03 

$ 

d 

1 

1 

1 

• 

T3  3 

o3  .i^ 

tj 

c3 

"*H       O 

£ 

0> 

£ 

•8 

8 

a 

>> 

1 

1 

•S§ 

'>  'Tj 

£ 

a 

a 

. 

S 

a 

a 

"3 

Time  period. 

Strain. 

tions  of  d( 

3 

°l 

d  *o 
fc-g 

1  « 

'II 
d  g 

i 

individua 
an  end  of 

e  minimu 

a 

individua 

rd  deviati* 

"o 

J 

| 

|| 

si 

"o 

j 

i 

03 

"o 

cs-Q 

^  c 

9 

"o  -s 

If   ' 

. 

. 

•a 

i 

§  g 

^  ** 

I 

O 

d 
55 

1* 

2^ 

ii 

d  8 

|| 

1  i 

II 
1* 

1 

1 

fi'J 

I 

|s 

Apr.     1,  1912- 

Plus  

8-21 

13 

4.6 

55 

4 

12 

257 

695 

495.4 

256.9 

23.4 

July  31,  1912 

Minus  .  . 

5-21 

I/ 

6.5 

71 

1 

29 

326 

717 

615.9 

272.6 

21.5 

-123.5 

32.0 

3  '.56 

Aug.     1,  1912- 

(  Plus  .... 

22-55 

36 

6.1 

220 

10 

9 

164 

440 

309.2 

209.8 

9.5 

+   30.7 

12.4 

2.48 

July  31,  1913 

(  Minus  .  . 

22-54 

35 

6.5 

229 

4 

153 

446 

275  5 

175  0 

7.9 

Aug.     1,  1913- 

Plus  .... 

56-100 

44 

6.9 

311 

31 

39 

197 

658 

406.4 

268.4 

10.3 

July  31,  1914 

Minus  .  . 

65-101 

4^ 

7.4 

349 

SO 

55 

210 

702 

439.2 

254.2 

10.3 

-32.8 

14  5 

2.26 

Aug.     1,  1914- 

Plus.  ... 

101-144 

41 

9.3 

383 

39 

52 

169 

724 

378.1 

268.4 

9.2 

+   15.8 

12l3 

l!28 

July  31,  1915 

Minus  .  . 

102-147 

45 

10.3 

A65 

79 

47 

170 

697 

362.3 

259.4 

8.1 

Test   series, 

Plus.  ... 

112 

16 

15.6 

249 

39 

8 

105 

738 

344.8 

222.9 

9.5 

+  'is!6 

12.9 

i!6i 

Oct.,  1914... 

Minus  .  . 

114 

16 

15.1 

259 

47 

15 

95 

734 

331.5 

215.1 

5.7 

Sept.    1,  1914- 

Plus.  ... 

105-116 

12 

10.4 

125 

17 

13 

167 

739 

397.1 

258.9 

15.6 

is.  9 

5.03 

Nov.  31,  1914 

Afiniis 

107-115 

12 

11  .5 

135 

25 

2 

107 

609 

301.9 

155.2 

10  6 

Test   series, 

Plus  ...'.' 

113 

35 

30.1 

1055 

244 

86 

108 

841 

383.1 

240.1 

4.98 

+   18.9 

7.72 

2.46 

Nov.  1914.  .  . 

Minus  .  . 

115 

35 

31.1 

1059 

277 

50 

96 

547 

364.2 

243.5 

4.95 

.... 

Oct.     1,  1914- 

Plus  .... 

109-121 

13 

11.2 

145 

24 

9 

130 

691 

352.5 

Dec.  31,  1914 

Minus  .  . 

111-123 

13 

11.9 

155 

36 

0 

97 

596 

350.2 

-  27.7 

Test   series, 

Plus  .... 

129 

36 

31.2 

1123 

242 

195 

96 

804 

421.9 

'287!  3 

"5".  78 

+  25.3 

's'.o' 

3  '.16 

Mar.,  1915    . 

Afinus 

131 

36 

31.5 

1133 

193 

151 

93 

396.6 

275.3 

5.52 

Feb.     1,  1915- 

Plus  .... 

125-135 

10 

10.2 

102 

6 

9 

101 

711 

317.5 

Apr.  30,  1915 

127-135 

12 

13  2 

159 

20 

8 

110 

529 

415.5 

—  95  3 

Aug.     1,  1915- 

Plus"*  .'  .' 

145-185 

37 

11.  « 

427 

36 

41 

107 

662 

301.1 

250.6 

8.2 

July  31,  1916 

Minus  .  . 

145-195 

3,9 

11.1 

434 

35 

35 

105 

725 

303.5 

222.1 

7.2 

'-"2.4 

10  9 

!«2 

Aug.     1,  1916- 

Plus  .... 

186-194 

7 

9.4 

66 

3 

16 

146 

762 

393.5 

227.1 

18.9 

+  63.8 

32^4 

1.97 

Sept.16,  1916 

Minus  .  . 

196-202 

7 

7.6 

53 

3 

* 

57 

562 

329.7 

254-4 

26.4 

TABLE  22. — Same-day  broods.    Summary  of  data  for  Line  714- 


1 

i 

s 

1 

i 

.1 

I 

£ 

d 

| 

c 

.2 

^"3 

I 

i 

a 

j 

03    g 

E 

ft 

3 

§ 

83   4J 

£ 

2 

M 

o 

§ 

a  '3 

to 

^ 

•3 

j 

1 

12 

I 

a 
a 

•a 

3 

a 

| 

|| 

1 

1 

Time  period. 

Strain. 

•s 

d 

.S 

:?^ 

'3 

I 

'? 

1 

o 
E 

*'i 

2 

1 

0 

o 

X2 

£ 

1 

N 

-3  * 

c  a 

"*   03 

f 

a 

| 

13 

E 

3 

§   03 

8 

JJ 

0 

g'o 

"S 

*o  •« 

03  £ 

2  2 

g  2 

i 

fe    00 

rQ 

S3  * 

d 

>  j5 

d 

Is3 

|£ 

>  •£ 

^ 

|| 

02 

o 

S* 

1 

§3 

Apr.  13,  1912- 
July  31,  1912 
Aug.    1,  1912- 
July  31,  1913 
Aug.    1,  1913- 
July  31,  1914 
Aug.     1,  1914- 
July  31,  1915 

(  Plus.  .    . 

\  Minus    . 
(  Plus..    . 
I  Minus    . 
(  Plus..    . 
I  Afinus    . 
(  Plus..    . 
l  Minus    . 

2 
2 
3 
3 
6 
6 
13 
13 

2.5 
6.5 
6.0 
2.7 
5.3 
5.5 
9.9 
12.7 

5 
IS 
18 
5 
32 
35 
129 
165 

0 

i 
i 

0 
3 

0 
19 

29 

7 
4 
0 
4 
6 
5 

190 
290 
340 
135 
275 
253 
117 
123 

I  1  0 

528 
790 
667 
225 
600 
675 
657 
700 
720 

438 
669 
429 
203 
292 
397 
292 
406 
342 

261.04 

274.24 
289.06 
91.12 
250.26 
269.45 
203.96 
259.43 

78.74 
51.  SO 
17.05 
21.73 
29.84 
SO.  7  2 
12.12 
15.20 

'  -231 
+226 

'93!  95 
27.62 

2!45 
8.18 

-105 
'  -114 

42.52 
'i9:44 

2.45 
5.  '56 

July  31,  1916 

(  Plus.  .    . 
I  Minus    . 

5 

5 

47 

6 

3 
12 

111 

155 

722 
739 

SSI 
403 

Sept.16,  1916 

\  Minus    . 

5.0 

45 

3 

S 

55 

627 

351 

seconds)  is  2.26  times  the  probable  error.  There  were  6  rather  small 
same-day  broods,  for  which  the  mean  for  the  minus  strain  was 
105  ±42.82  seconds  the  larger.  Again,  as  compared  with  the  pre- 


58 


SELECTION   IN    CLADOCERA   ON   THE   BASIS   OF 


2.OO 

1.50 

l.OO 

.50 

00 


ceding  long  period,  there  is  a  reversal  in  this  longer  period  of  the 
direction  of  the  difference  in  the  mean  reaction-times  for  the  entire 
data  and  in  that  for  the  same-day  broods. 

For  the  next  year-period  (August  1,  1914- July  31,  1915),  the. 
means  were  378.1  and  362.3  seconds.     The  difference,  which  was 
+  15. 8  ±12. 3  seconds,  is  not  statistically  significant  and  is  again  in 
the  reverse  direction  from  that  for  the  preceding  period.     During 
this  year-period  the  same-day-brood  data  consisted  of  13  broods  and 

4-5       8-9      12-1       4-5       8-9      12-1      4-5       8-9  ^  12-1       4-5       8-9      12-1       4-5      8~9 


1912 


1913 

A 


1914- 


-I 


T 

til 


?  1915 

8s 


il 


1916 


Til    B  T  , 


t      lit      , 


I- ]     'H| 


.75 
.5C 
.25 
00 
.25 
.50 
.75 


60O 


4-50 


3OO 


150  h      1912 

4-5      8-9     12-1 


1913 
4-5      8-9 


12-1 


1914- 
4-5      8-9      12-1 


1915 
4-5      8-9 


12-1 


1916       . 
4-5      8-9 


FIGURE  7. — Line  714. 

A.  Reproductive  indices,  actual  values. 

B.  Reproductive  indices,  superiority. 

C.  Reaction-time  curves. 

the  mean  for  the  minus  strain  was  114  ±19.44  seconds  higher  than 
that  for  the  plus  strain.  This  is  a  difference  almost  6  times  the 
statistical  probable  error.1 

A  test  series  conducted  during  October  1914,  containing  249 
individuals  in  the  plus  strain  and  289  in  the  minus  strain,  gave 

1  These  same-day  broods  are  pretty  well  scattered  through  the  year  period,  except  that  none 
occurred  during  the  last  4  months.  The  averages  obtained  for  the  same-day  broods  are  more 
dependable  than  the  averages  for  the  data  as  a  whole  when  there  are  sufficient  numbers,  and  129 
and  1G5  individuals  constitute  fairly  good  numbers.  It  is  unusual  that  the  reaction-time  means 
for  the  complete  data  and  for  the  same-day-brood  data  for  the  same  period  should  differ  so  widely 
as  in  the  present  case. 


A   PHYSIOLOGICAL   CHARACTER.  59 

means  of  344.8  seconds  for  the  plus  strain  and  331.8  seconds  for  the 
minus  strain.  The  difference,  +13.0  ±12.9  seconds,  is  not  of  statis- 
tical significance.  A  second  test  series  conducted  in  November  1914 
contained  1,055  individuals  in  the  plus  strain  and  1,089  in  the  minus 
strain.  The  difference  in  the  mean  reaction-time  was  +18. 9  ±7. 72 
seconds,  2.46  times  the  probable  error.  A  third  test  series  was  con- 
ducted during  March  1915.  The  numbers  were  large,  1,123  and  1,133 
individuals  in  the  two  strains.  The  mean  reaction-times  were  421.9 
and  396.6  seconds.  The  difference  was  +25. 3  ±8.0  seconds  and  is 
3.16  times  the  probable  error. 

It  will  be  seen  that  there  is  a  rather  striking  similarity  in  the 
differences  obtained  for  the  3  test  series  and  for  the  entire  selection 
data  for  this  year-period,  the  differences  being  +13.0,  +18.9,  and 
+25.3  for  the  3  test  series  and  +15.8  seconds  for  the  selection  data 
as  a  whole.  Such  harmonious  results  seem  to  indicate  that  the  plus 
strain  of  Line  714,  during  this  year-period,  was  actually  less  reactive 
in  spite  of  selection,  although  the  same-day-brood  data  oppose  this 
conclusion. 

For  the  next  year-period  of  selection  with  Line  714  (August  1, 
1915-July  31,  1916)  the  means  were  301.1  and  303.5  seconds.  The 
difference  was  —2. 4  ±10. 9  seconds.  There  remains  a  short  period 
of  1J/2  months  during  which  selection  was  continued.  There  were 
only  66  individuals  in  the  plus  and  53  individuals  in  the  minus  strain. 
The  difference  (+63. 8  ±32.4)  is  not  statistically  significant. 

Considering  the  data  for  Line  714  as  a  whole,  it  is  clear  that 
there  is  not  an  effect  of  selection. 

Figure  7c,  showing  the  reaction-time  curves  for  the  two  strains 
of  Line  714,  presents  curves  with  large  minor  irregularities.  It  will 
be  noted  that  throughout  the  first  6  months  of  selection  with  Line 
714  there  is  a  considerable  divergence,  the  minus  strain  having  much 
the  higher  reaction-time.  Such  consistencies  in  reaction-time  differ- 
ences for  limited  periods  also  occur  in  Line  695  for  a  period  of  8 
months  and  in  Line  740  for  a  period  of  10  months.  It  would  seem 
that  these  are  more  than  mere  chance  results,  but  in  each  of  these  3 
cases  further  selection  not  only  did  not  result  in  increasing  the 
divergence,  but  in  all  3  of  these  cases  the  divergence  disappeared. 
Assuming  for  the  moment  that  an  effect  of  selection  was  present  in 
these  cases,  it  was  later  lost;  but  in  what  manner  this  loss  could 
have  occurred  is  not  clear.  Selections  were  not  relaxed,  and  there 
seems  to  be  no  reason  to  suppose  that  a  mutation  had  affected  one 
of  the  strains  in  each  of  these  3  lines. 

However  suggestive  of  an  effect  of  selection  the  early  portion  of 
the  curves  for  Line  714  are,  it  is  clear  from  the  remainder  of  the 
curves  that  a  selective  effect  is  not  present  (figure  7c).  The  detailed 
analysis  of  the  data  has  already  shown  this. 


60  SELECTION   IN    CLADOCERA    ON   THE    BASIS   OF 

The  minus  strain  of  Line  714  was  on  the  whole  slightly  more 
vigorous  than  the  plus  strain.1  The  plus  strain  was  superior  in  vigor 
during  13  two-month  periods;  the  minus  strain  more  vigorous  during 
14  two-month  periods.  The  differences  in  favor  of  the  minus  strain 
were  somewhat  greater  than  those  in  favor  of  the  plus  strain. 

There  is  no  apparent  relation  between  vigor  and  reaction-time. 
Casual  inspection  of  the  reaction  curves  and  the  reproductive  index 
figure  (figure  7)  gives  this  impression,  and  the  following  facts  bear 
it  out:  During  11  periods  the  temporarily  more  vigorous  strain  was 
the  more  reactive,  during  15  periods  the  more  vigorous  strain 
the  less  reactive,  and  during  1  period  the  reaction-times  were  th< 
same.  Selecting  the  9  periods  during  which  the  mean  reproductive 
indices  differed  by  more  than  0.30,  it  is  found  that  in  5  periods  th< 
more  vigorous  strain  is  the  less  reactive,  in  3  periods  the  moi 
vigorous  strain  is  the  more  reactive,  and  for  1  period  the  mean 
action-times  were  the  same.  Selecting  the  9  two-month  perio< 
during  which  the  reaction-time  means  differed  by  more  than  II 
seconds,  it  is  found  that  in  2  cases  these  large  differences  in  the  me* 
reaction-time  correspond  with  periods  in  which  the  more  reactr 
strain  was  also  the  more  vigorous,  but  that  in  7  cases  the  more  r< 
active  strain  was  the  less  vigorous  of  the  two. 

There  is  an  interesting  feature  of  the  curves  (figure  7c)  in  th< 
marked  influence  of  environment  upon  the  mean  reaction-time  of  the 
two  strains.  In  spite  of  considerable  irregularities  of  the  curves  for 
the  two  strains,  they  follow  each  other  to  a  remarkable  extent. 
Nothing  other  than  environmental  influences  would  seem  to  account 
for  such  a  coincidence. 

LINE  719. 

The  data  for  Line  719  will  be  found  in  tables  23,  24,  25,  and  26 
and  in  figure  8.  This  is  one  of  the  original  lines  of  Daphnia  pulex 
taken  into  the  laboratory  in  November  1911.  Selection  was  begun 
(March  28,  1912)  in  the  eighth  laboratory  generation  and  continued 
for  130  and  138  generations,  until  both  strains  were  lost  in  June  1915. 

Consultation  of  the  curves  (figure  8c)  shows  that  there  is  no 
effect  of  selection;  some  interesting  features  of  this  data,  however, 
will  be  brought  out  by  a  somewhat  detailed  analysis. 

For  the  first  4  months  the  mean  reaction-times  for  the  plus  and 

minus  strains  were  450.1   and  493.1   seconds.     The  difference  is 

-  43  ±  31.8  seconds  (table  25).    For  the  same-day  broods  (only  3)  the 

corresponding  data  are  718  and  539  seconds,  the  difference  being 

+  179  ±76.00  seconds. 

1  The  average  reproductive  index  (obtained  by  dividing  the  sum  of  the  reproductive  indices 
by  two-month  periods  by  the  number  of  two-month  periods)  for  the  plus  strain  was  1.006,  for 
the  minus  strain  1.034. 


A    PHYSIOLOGICAL   CHARACTER.  61 

TABLE  23. — Selection  data  summarized  by  two-month  periods  for  Line  719  plus. 


Time  period. 

C 
0 

1 

m 
O 

a 

0 

£ 
"3 
6 
fc 

Total  No.  of  young  tested. 

1  Average  No.  of  young  per 
first  brood. 

O  •£ 

a  „ 

111 
$'*•* 

Average  No.  of  individuals 
per  first  brood  divided  by 
average  age  of  mothers. 

No.  of  negatively  reacting  in- 
dividuals. 

3  _ 

«  cd 

il 
is 

o-g 

Average  minimum  reaction-  1 
time.  1 

| 

1 
3 

3 

1 

h 

sj 

i 

t* 

*3 
3 

3 
OT 

Mean  individual  reaction- 
time. 

3 

1 
1 

1 

d 

3 
"3 

3 

Apr.-May  1912... 
June-July  1912... 

Aug.-Sept.  1912.  .  . 
Oct.-Nov.  1912.  .  . 
Dec.  1912-Jan.  1913 
Feb.-Mar.  1913... 
Apr.-May  1913... 
June-July  1913.  .  . 

Aug.-Sept.  1913.  .  . 
Oct.-Nov.  1913.  .  . 
Dec.1913-  Jan.1914 
Feb.-Mar.  1914.  .  . 
Apr.-May  1914... 
June-July  1914... 

Aug.-Sept.  1914... 
Oct.-Nov.  1914  .  .  . 
Dec.  1914-  Jan.  1915 
Feb.-Mar.  1915... 
Apr.-May-June  '15 

9-13 
14-19 

20-21 
21-24 
25-31 
32-38 
39-45 
46-54 

55-62 
63-68 
69-75 
76-81 
82-90 
91-96 

97-104 
105-113 
114-121 
122-127 
128-137 

8 
6 

2 

4 
7 

7 
7 
9 

8 
5 
7 
6 
9 
6 

8 
9 
8 
5 
10 

27 
51 

3 

16 
67 
51 
47 
57 

86 
37 
48 
32 
81 
31 

73 

108 
65 
59 
79 

6.8 
6.8 

1.5 
2.3 
10.2 
7.3 
6.7 
6.0 

9.2 
6.3 
6.2 
5.6 
8.5 
4.6 

9.6 
12.2 
8.1 
12.5 

8.4 

9.6 
11.4 

11.5 
12.8 
9.1 
8.5 
8.3 
7.2 

6.3 
7.5 
8.4 
8.8 
7.1 
7.2 

7.1 
7.4 
7.9 
9.1 
8.2 

0.71 
.60 

.13 
.18 
1.12 
.86 
.81 
.83 

1.46 

.84 
.74 
.64 
1.20 
.64 

1.35 
1.65 
1.03 
1.37 
1.02 

0 
4 

0 
0 
2 

1 
1 
2 

9 

1 
0 
13 
6 

11 
24 
3 
9 
6 

16 
3 

0 
0 
6 
5 
0 
12 

7 
0 
9 
3 
11 
9 

13 
2 
9 
7 
18 

214 
240 

288 
220 
115 
147 
111 
173 

105 
141 
319 
233 
186 
326 

165 
87 
119 
137 
315 

534 
635 

295 
448 
476 
437 
316 
595 

539 
394 
725 
626 
801 
814 

834 
517 
698 
623 
768 

15738 
19370 

825 
4138 
18737 
16685 
9910 
22603 

31176 
9044 
22299 
14002 
34353 
18139 

33103 
28021 
24354 
20971 
33398 

583 
380 

275 
259 
280 
327 
211 
397 

363 
244 
465 
438 
424 
585 

453 
259 
375 
355 
423 

224.17 

21.17 

251.87 
258!  36 

18.32 

25!  is 

248.35 

30.09 

297.34 

22.57 

TABLE  24. — Selection  data  summarized  by  two-month  periods  for  Line  719  minus. 


i 

• 

S& 

j 

3 

-, 

1 

1 

S 

i 

1 

1 

S 

ca  * 

I 

jf-g 

o3 

M 

1 

3 

a  . 

i 

1 

8 

1 

c 

+> 

si 

1 

*4-t      fll 

£ 

£ 

8 

£ 

-3 

1 

IB 

* 

Time  period. 

"3 

| 

M 
C 
3 
0 

•3 
6 

1 

•3 

0 

2 

•J 

No.  of  your 
livided  by  a1 
lers. 

1 

ndividuals 
n  end  of  th 

minimum 

maximum 

1 

"3 

1  deviation 

s  error  of  m 

;e  between  i 
d  minus  stn 

!  error  of  di 

I 

ot 

* 

3 

1 

|° 

11 

**-rj  "o 

.  "> 

*1 

!«• 

a>    S 

•3 

3 

1.1 

1 

c  o 

1 

•§ 

if 

O 

6 

o 

> 

>'5 

£*  _Q  *O 

O  '^ 

d  « 

>•! 

>-J 

;3 

^•J 

* 

2 

*Q. 

2 

5 

0 

Z 

H 

•< 

•«"* 

^ 

* 

* 

<^ 

•<•* 

03 

s 

« 

0 

£ 

Q 

)->r.-May   1912... 

9-14 

7 

37 

6.2 

9.1 

0.68 

o 

14 

292 

896 

21155 

572 

+11 

.ne-July   1912..  . 

15-21 

8 

39 

5.6 

8.5 

.66 

o 

7 

210 

719 

16320 

418 

-38 

l4g.-Sept.  1912... 

22-24 

1.3 

17.5 

.07 

o 

o 

200 

251 

854 

214 

+61 

't.-Nov.  1912.  .  . 

25-28 

6 

15 

1.5 

19.5 

.08 

o 

o 

184 

239 

3809 

254 

+5 

f^c.  1912-Jan.  1913 

29-36 

8 

6.9 

8.4 

.82 

0 

0 

93 

333 

12566 

228 

+52 

tb.-Mar.  1913..  . 

37-42 

6 

35 

5.8 

9.0 

.64 

0 

0 

126 

353 

7938 

227 

ii3.'26 

12.91 

+  100 

"24.7Q 

4!o3 

t>r.-May   1913 

43-50 

8 

)0 

7  5 

7  4 

1  00 

g 

o 

124 

423 

15220 

254 

-43 

Ine-July    1913.'  '.'. 

51-58 

10 

41 

4^6 

7^2 

!64 

4 

1 

255 

492 

13543 

330 

+67 

— 

Ltg.-Sept.  1913... 

59-67 

9 

74 

8.2 

7.0 

1.17 

8 

4 

101 

634 

18502 

250 

196.73 

15.43 

+  113 

23.94 

4.72 

V  t.-Nov.  1913 

68-74 

7 

06 

8  0 

8.  1 

99 

o 

3 

149 

403 

14036 

251 

—7 

itc.1913-Jan.1914 

75-81 

7 

58 

8.'  3 

8.3 

i.'oo 

4 

5 

179 

701 

21682 

374 

207  '.78 

18.40 

+91 

31.17 

2.91 

-b.-Mar.  1914.  .. 

82-89 

7 

40 

5.7 

9.0 

.63 

2 

255 

722 

16870 

422 

+  16 

i*»r.—  May   1914. 

Q(X_Q7 

QA 

57 

6n 

00 

4 

00 

ooq 

837 

36841 

439 

—  15 

line-July   1914.'.'! 

yu  y/ 
98-105 

8 

04: 

63 

.   9 

7.8 

.  y 

7.8 

.  oo 

1.00 

10 

KC 

9 

A£i\7 

210 

791 

29417 

467 

24ili8 

20.50 

+  118 

36.40 

3.24 

fcig.-Sept.  1914.  . 

[06—112 

7 

50 

6.6 

7  3 

.90 

15 

g 

206 

769 

20071 

401 

+  52 

Ht.-Nov.  1914 

L  13-120 

g 

33 

95 

74 

l!28 

20 

5 

183 

646 

22045 

266 

-7 

P:c.l914-jan.l915 

121-128 

57 

10^9 

7^8 

1.39 

9 

5 

168 

697 

31633 

364 

+  11 

Db.-Mar.  1915. 

[29—135 

7 

70 

9  5 

8  5 

1   12 

g 

7 

116 

639 

26135 

373 

-18 

Kr.,May,JuDe,1915 

136-145 

9 

65 

8.1 

7~.8 

li04 

19 

6 

147 

606 

22580 

347 

257!  35 

21.53 

+  76 

31.18 

2.43 

62 


SELECTION   IN    CLADOCERA   ON   THE    BASIS   OF 


During  the  year  (August  191 2- July  1913)  the  means  were 
302.5  and  256.3  seconds.  The  difference  (+46.2  ±12.1  seconds)  is 
3.81  times  the  probable  error,  a  difference  statistically  significant. 
For  the  12  same-day  broods  the  means  are  314  and  254  seconds— 
and  the  difference  is  +60  d=  21.68  seconds,  2.76  times  the  probable 
error. 

TABLE  25. — Selection  summary  for  Line  719. 


& 

A 

o 

3 

i 

a 

£ 

i 

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a     j 

-a  "o 

!  s 

i 

60  -ii 

.2 

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£1 
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"3  .2 
3  "S 

"8 

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1. 

§ 

d 

a 

§ 

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d 

o 

a 

S 

a 

1 

| 

Time  period. 

Strain. 

"U 
0 

1 

*_  ° 

6? 

"*    C 

"« 

"?-    M-. 

3    0 

1-s 

inimur 

3 

S 

'i 

3 
| 

3 

g 

S 

i< 

"8 

1 

1 

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—    cj 

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0 

§«>£ 

—  i  o 

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c?  a5 

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(3  v 

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2  § 

u   oj 

a 

6 

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d  1 

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1 

fS-2 

O 

<  ft 

H  * 

S^ 

Z  *• 

^~ 

«!~ 

^^ 

M 

£ 

Q  * 

& 

Q-° 

Apr.       1912- 
July  31,  1912 

(  Plus.  ... 
I  Minus  .  . 

9-19 
9-21 

14 

15 

5.6 
6.1 

78 

4 

0 

19 

225 
£57 

577 

450.1 
493.1 

275.3 

308.4 

21.0 

-43  !o 

•u-y 

/.'35 

Aug.    1,  1912- 

(  Plus  

20-54 

36 

6.7 

241 

6 

23 

156 

454 

302.5 

232.3 

lo'.i 

+46.2 

12^1 

3.81 

July  31,  1913 

I  Minus  .  . 

22-58 

41 

5.1 

S10 

/O 

y 

ISA 

37£ 

256.3 

142.9 

6.6 

Aug.  1,  1913- 

(  Plus.... 

55-96 

41 

7.7 

315 

31 

39 

215 

664 

409.6 

257.6 

9.8 

+43.3 

13.2 

3  '.28 

July  31,  1914 

I  Minus  .  . 

59-105 

46 

8.2 

375 

28 

45 

183 

0S5 

366.3 

255.8 

8.9 

Test   series, 

(  Plus  .... 

89 

23 

13.9 

320 

96 

82 

156 

879 

520.8 

289.2 

10.9 

+57.2 

14.9 

3^83 

May    1914  .  . 

I  Minus  .  . 

96 

23 

14-4 

331 

101 

60 

754 

SOS 

463.6 

271.9 

10.1 

Apr.     1,  1914- 

(  Plus  .... 

82-93 

12 

8.2 

98 

15 

16 

224 

826 

453.7 

+  19.2 

JuneSl,  1914 

(  Minus  .  . 

90-101 

12 

9.0 

/OS 

5 

£3 

794 

434.5 

Aug.     1,  1914- 

(  Plus.  ... 

97-137 

40 

9.6 

384 

53 

49 

172 

693 

364.2 

264.9 

9.1 

+  19.2 

12.5 

1.53 

June  25,  1915 

\  Minus  .  . 

106-145 

39 

9.1 

355 

71 

&Q 

1(54 

668 

345.0 

239.8 

8.6 

Test   series, 

j  Plus  .... 

110 

43 

22.8 

979 

189 

46 

95 

688 

310.9 

217.2 

4.7 

+  20.3 

6.3 

3.22 

Nov.   1914.  . 

I  Minus  .  . 

118 

43 

£3.7 

1019 

£17 

35 

103 

697 

290.6 

197.0 

4.£ 

Oct    1     1914- 

(    Plug 

105-117 

13 

9.9 

129 

27 

3 

93 

511 

261.1 

Dec.  31,  1914 

I  Minus  .  . 

113-124 

12 

11.1 

£tf 

<5 

-27.0 

TABLE  26. — Same-day  broods.    Summary  of  data  for  Line  719. 


S3 

i 

3 

d 

2 

A 

P, 

03 

,0 

to 

M.X 

a 

a 

a 

3 

d 

II 

i 

i 

S 

a' 

s 

o. 

O 

•g 

1 

«! 

s 

s 

B 

*O 

i 

if 

•3 

1 

O 
1* 

1 

j 

"«  «*. 

3    0 

3 

3 

1 

!§ 

"o 

1  i 

"8 

12 

Time  period. 

Strain. 

1 

1 

•a 

L 

i! 

'S 

1 

•fl 

S 

1 

G 

T) 

!j 

.2 

Sfe 

v& 

&~i 

«»* 

G    S 

•^  Qj 

M 

Si 

S 

— 

M 

C    u 

"8 

11 

•o 

"82 

"o"? 

2  § 

g  « 

C   o3 

1 

Jo 

«  § 

1 

»  « 

6 

6 

»2  =5 

d  a> 

I'-3 

>  -^ 

S  ^ 

5 

2 

"E 

f 

S  2 

^ 

* 

* 

7* 

55 

•< 

^ 

00 

* 

PH 

Q 

Apr.     8,  1912- 

(  Plus.  ... 

3 

3.3 

10 

0 

5 

557 

703 

718 

257.80 

54.99 

+  179 

76.00 

2.35 

July  31,  1912 
Aug.     1,  1912- 

(  Minus  .  . 
(  Plus.  ... 

9 

12 

5.7 
7.3 

17 
88 

0 
2 

12 

££S 
141 

465 

539 
314 

M0.76 

262.11 

8*.  A7 

18.85 

'+60' 

2l!68 

2:76 

July  31,  1913 

I  Minus  .  . 

It 

(7.9 

S3 

S 

1 

45/ 

£54 

144.^ 

10.71 

Aug.     1,  1913- 

<  Plus  .... 

11 

7.9 

87 

8 

4 

174 

624 

340 

207.54 

15.01 

July  31,  1914 

I  Minus  .  . 

S.O 

88 

£04 

3.94 

£49  .  6£ 

17.95 

'  -54' 

£3l39 

•^.  50 

Aug.     1,  1914- 

(  Plus.... 

10 

11.3 

113 

14 

11 

114 

741 

351 



June25,  1915 

(  Minus  .  . 

10 

7.S 

7S 

/£ 

7 

£5£ 

731 

37S 



'  -£7 

For  the  following  year  (August  1913-July  1914)  the  means  are 
409.6  and  366.3  seconds.  The  difference  is  +43.3  ±13.2  seconds 
and  is  3.28  times  the  probable  error.  Again  the  plus  mean  is  larger 
by  a  margin  of  statistical  value.  The  11  same-day  broods  have 


A   PHYSIOLOGICAL   CHARACTER.  63 

means  (340  and  394  seconds)  differing  by  -54  ±23.39  seconds,  a 
result  out  of  accord  with  that  for  the  means  for  the  complete  data 
for  this  year  and  with  that  for  the  test  series  now  to  be  considered. 

The  test  series  conducted  in  May  1914  consisted  of  23  broods 
from  each  strain  (320  individuals  of  the  plus  and  331  individuals  of 
the  minus  strain).  Of  the  23  pairs  of  broods,  17  broods  from  the  plus 
strain  had  the  higher  reaction-time,  while  the  minus  brood  had  the 
higher  reaction-time  in  only  6  cases.  The  reaction-time  means  for 
the  entire  test  series  were  520.8  and  463.6  seconds.  The  difference 
was  +57.2  ±14.9  seconds,  a  difference  3.83  times  the  probable  error 
(table  25). 

For  the  final  period  (11  months)  of  the  experiment  with  Line 
719,  the  mean  for  the  plus  strain  was  364.2  seconds,  for  the  minus 
strain  345.0  seconds.  The  difference  was  +19.2  ±12.5  seconds,  a 
difference  only  1.53  times  the  probable  error.  The  ten  same-day 
broods  gave  means  of  351  and  378  seconds,  and  a  difference  of  —27 
seconds;  again  the  same-day-brood  data  is  out  of  accord  with  the 
data  for  the  year-period.  A  test  series  conducted  during  November 
1914  and  consisting  of  nearly  1,000  individuals  from  each  of  the 
strains  gave  means  of  310.9  and  290.6  seconds  and  a  difference  of 
+20.3 ±6.3  seconds  (table  25).  This  difference,  while  small,  is  3.22 
times  the  statistical  probable  error. 

The  course  of  the  curves  (figure  8c)  throughout  is  rather  irregu- 
lar, particularly  for  the  plus  strain,  but  the  marked  tendency  for  the 
curve  for  the  plus  strain  to  be  higher  than  that  for  the  minus  strain 
is  suggestive  of  a  real  difference  in  reactiveness  between  these  two 
strains,  with  the  minus  strain  generally,  though  slightly,  the  more 
reactive.  In  addition  to  the  data  for  the  entire  course  of  the  experi- 
ment when  considered  by  longer  periods,  the  two  test  series,  and  a 
minor  portion  of  the  data  for  the  same-day  broods,  likewise  tend  to 
indicate  that  the  minus  strain  was  the  more  reactive.  But  the 
differences  are  less  pronounced  during  the  last  year  of  the  experiment 
than  during  the  earlier  two  years.  The  minus  strain  was  more  re- 
active than  the  plus  strain  by  only  19  seconds  (a  difference  not  of 
statistical  value)  during  the  last  year-period.  The  differences  for  the 
earlier  two  years  were  46  and  43  seconds  and  were  3.81  and  3.28 
times  their  probable  errors.  However,  the  large  test  series  conducted 
during  this  last  year-period  indicated  that  the  minus  strain  was  the 
more  reactive  by  20.3  seconds  (a  difference  very  near  that  for  the 
last  year's  selection  data)  and  because  of  the  large  number  of  indi- 
viduals used  in  this  series  and  the  consequently  smaller  probable 
error  this  difference  was  3.22  times  the  probable  error. 

The  data  for  Line  719  as  a  whole  and  particularly  the  data  of 
the  2  test  series  are  very  suggestive  of  a  genetic  difference  in  oppo- 
sition to  selection  between  the  two  strains  of  Line  719,  in  spite  of  the 
lack  of  confirmation  from  most  of  the  same-day-brood  data  and  the 


64 


SELECTION   IN    CLADOCERA   ON   THE    BASIS   OF 


reduction  in  amount  of  this  difference  during  the  last  year  of  the 
experiment. 

Additional  evidence  of  a  significantly  lowered  reaction-time  for 
the  minus  strain  of  Line  719  is  obtained  by  comparison  with  the 
minus  strains  of  the  other  D.  pulex  lines  which  showed  no  evidence 
of  modification  in  reactiveness  during  the  selection  experiments, 
Lines  691,  695,  713,  714,  and  751.  A  composite  curve  for  the  re- 
action-time mean  for  the  minus  strains  of  these  lines  compared  with 


1.50 
1.00 
.50 


00 


8  I 


4-5          8-9          12-1  4-5          8-9          12-1  4-5          8-9          12-1  4-6 

1912  1913  1914  1915 


:..  .,! 

T 

B 

T 

T 

j 

i 

f! 

I.' 

.50 
.25 
00 
.25 
.50 

O 

J 

* 

1 

i 

i 

- 

_4-5          8-9          12-1  4-5          8-9          12-1  4-5          8-9          12-1          4-6 


600  h 


450 


300 


150 


FIGURE  8. — Line  719. 

A.  Reproductive  indices,  actual  values. 

B.  Reproductive  indices,  superiority. 

C.  Reaction-time  curves  with  a  superimposed  curve  (in  faint  broken  line)  representing  the 

combined  reaction-time  means  for  the  minus  strains  of  all  Daphnia  pulex  lines  in  which 
significant  reaction-time  differences  probably  did  not  arise. 

the  mean  for  the  minus  strain  of  Line  719  (figure  8c)  shows  that 
beginning  with  August  1912  and  continuing  to  January  1914  the 
mean  for  the  minus  strain  of  Line  719  was  lower  than  that  for  the 
combined  mean  for  the  other  minus  strains  for  the  same  two-month 
periods  in  8  of  9  two-month  periods;  6  of  these  differences  (ranging 
from  48  to  197  seconds)  were  of  statistical  value  and  one  other  just 
escaped  statistical  significance  because  of  the  small  numbers  of 
reaction-time  records  involved.  From  February  1914  the  minus 


A   PHYSIOLOGICAL   CHARACTER.  65 

strain  of  Line  719  continued  more  reactive  than  the  combined  means 
of  the  other  lines  for  10  months,  but  the  differences  were  small  and 
not  of  statistical  value.  For  the  remaining  7  months  of  the  curve  the 
differences  were  variable  and  the  mean  for  the  plus  strain  did  not 
differ  appreciably  from  the  composite  curve  of  means  for  the  corre- 
sponding plus  strains. 

The  evidence  seems  suggestive  of  a  mutation  in  the  minus  strain 
of  Line  719,  rendering  it  the  more  reactive  to  light.  Since  the  differ- 
ence between  the  mean  for  the  minus  strain  of  Line  719  and  the  plus 
strain  of  the  same  line,  as  well  as  between  the  Line  719  minus  strain 
and  the  combined  means  for  the  minus  strains  of  the  lines,  which 
may  fairly  serve  as  checks,  later  decreased  and  was  apparently 
practically  lost,  it  seems  probable  that  selection  served  to  reduce  or 
eliminate  the  effect  of  this  apparent  mutation. 

This  reduction  in  (or  possible  elimination  of)  the  reaction-time 
difference  in  Line  719  is  worthy  of  consideration  as  contrasted  with 
Line  757,  in  which  the  difference  in  reaction-time  (which  in  that 
case  is  in  the  direction  attempted  in  selection)  became  larger  and 
larger  in  successive  year-periods,  until,  instead  of  being  a  difference 
of  5.5  per  cent,  as  in  the  last  period  of  selection  in  Line  719,  it  was 
99  per  cent  in  the  last  year-period  for  Line  757. 

Line  719  affords  a  case  parallel  to  that  of  Line  689,  in  which  the 
plus  strain  likewise  persistently  had  a  higher  reaction-time,  contrary 
to  any  influence  of  selection.  With  Line  689  the  period  of  the  experi- 
ment was  much  shorter,  but  otherwise  the  results  are  very  similar, 
except  that  in  Line  719  the  difference  is  not  so  large  or  so  uniform, 
is  not  supported  by  most  of  the  same-day-brood  data,  and  became 
much  smaller  during  the  latter  part  of  the  experiment. 

It  is  to  be  noted  with  Line  719  that  in  reproductive  vigor  the 
minus  strain  was  in  general  inferior  to  the  plus  strain.  This  was 
markedly  true  during  the  first  and  the  last  year-periods.  The  minus 
strain  was  slightly  superior  in  vigor  during  the  second  year-period. 
Yet  the  minus  strain  was  the  more  reactive  throughout  all  these 
year-periods  and  was  (absolutely)  somewhat  less  reactive  during  its 
year  of  superior  vigor  (August  1913-July  1914)  than  during  the  other 
two  years. 

The  curves  for  the  mean  reaction-times  by  two-month  periods 
(figure  8c)  indicate  an  interesting  correlation  between  the  means  for 
the  two  strains,  the  plus  and  minus  curves,  in  spite  of  their  fluctua- 
tions, following  each  other  remarkably  closely.  This  is  again  an 
expression  of  environmental  effect.  The  small  and  variable,  though 
fairly  pronounced,  superiority  in  reactiveness  on  the  part  of  the 
minus  strain  for  most  of  the  period  of  selection  is  rendered  even  more 
interesting  by  the  fact  that  the  differences  persist  while  the  curves 
sweep  up  and  down  under  environmental  influences. 


66 


SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 


LINE  751. 

Line  751  was  obtained  in  October  1912  from  the  surface-water 
pond  on  the  upland  (Pond  I),  from  which  Line  689  came  a  year 
earlier.1  Selection  was  begun  at  once.  This  is  the  only  line  of  Daph- 
nia  pulex  with  which  selection  was  not  begun  during  March  1912, 
after  having  been  in  the  laboratory  from  5  to  8  generations  before 

TABLE  27. — Selection  summary  for  Line  751. 


•3  . 

\ 

c 

o 

1 

§ 

ft 

d 

i 

S 

i 

11 

J  § 

H 

G 

c  c 

o 

•^ 

S 

§ 

"g 

•*> 

II 

"o 

1: 

s 

S 

£ 

p 

d 

1 

?a 

>! 

1 

i 

•2  g 

3  J 

i 

<n  5 

s 

s 

a 

§ 

a 

•9 

Tl 

Time  period. 

Strain. 

•o 

roods  tes 

"sl 

l| 

X   rt 

j>> 

'v 

i 

ndividua] 
n  end  of 

minimui 

maximu 

ndividua 

i 

8 

T3 

i  error  of 

j| 

g| 

i 

« 

1 

1 

JD 

"8 

|l 

I* 

S| 

•8| 

li 
" 

S3  I 

li 

"5 

1 

V 

"2 

c  > 

1 

|! 

O 

6 

l& 

I15 

1^ 

°£ 

l« 

5 

02 

I 

QS 

1 

C3  ^ 

S-0 

Nov.  13,  1912- 

C  Plus.... 

1-31 

32 

8.2 

261 

10 

8 

139 

450 

282.2 

179.6 

7.73 

+  11.4 

10.28 

1  10 

July  31,  1913 

\  Minus  .  . 

1-31 

S3 

s.i 

15 

J^5 

270.8 

6.78 

Aug.   1,   1913- 

(  Plus  .... 

32-71 

40 

8.0 

321 

14 

31 

177 

657 

375.4 

252^7 

9.51 

June  11,  1914 

I  Minus  .  . 

32-70 

40 

8.3 

SSI 

27 

400.7 

256.7 

9.62 

-25'.S 

13.46 

/  ss 

Test    series, 

(  Plus.  ... 

54 

17 

19.3 

328 

19 

41 

140 

752 

416.1 

260.0 

9.7 

Jan.     1914 

'  Minus  .  . 

63 

17 

10.2 

327 

16 

60 

UO 

738 

433.9 

267.7 

10.0 

-17.8 

"lS.9 

;!«« 

Dec.    1,  1913- 

(  Plus  .... 

48-58 

11 

8.0 

88 

3 

10 

207 

688 

414.0 

233.0 

16.8 

Feb.  28,  1914 

I  Minut  .  . 

47-57 

11 

04 

1 

£50 

#?*.7 

252.6 

-74.7 

27.10 

£.75 

TABLE  28. — Same-day  broods.    Summary  of  data  for  Line  751. 


Time  period. 


Nov.  13,  1912- 
July  31,  1913 

Aug.  1,  1913- 
June 11, 1914 


Strain. 


Plus.... 
Minus  . . 
Plus.... 

Minus  . . 


7.4 

U 


119 

136 

88 

ni 


N 
II 


I! 

00  J3 

K 


Avera 
time. 


166 
165 
165 
10S 


Aver 
tim 


489 
475 
565 
676 


325 

£55 
350 


204.06 
170.04 


12.62 


II 

1l 

ll 
Qft 


+37 


+5 


ble 


16.022.30 


the  beginning  of  selection.  Selection  was  continued  for  almost  20 
months,  when  both  strains  were  lost  (in  the  seventy-first  and  seven- 
tieth generations).  The  data  for  this  line  are  given  in  tables  27  and 
28  and  figure  9. 

For  the  first  longer  period  of  selection  (9  months)  the  mean 
reaction-time  for  the  plus  strain  was  282.2  seconds,  for  the  minus 
strain  270.8  seconds.  The  difference  (+11.4  ±10.28)  was  not 
significant  (table  27).  There  was  a  large  number  (16)  of  same-day 

1  All  the  lines  of  D.  pulex  except  689  and  751  came  from   Pond  II,  the  spring-fed  pond  in 
the  woods. 


A50   - 


300 


150 


00 


A   PHYSIOLOGICAL   CHARACTER.  67 

broods.  The  mean  reaction-time  for  those  of  the  plus  strain  was 
325  seconds,  for  the  minus  strain  288  seconds.  The  difference 
(+37  ±16. 02  seconds)  is  in  the  same  direction  as  that  for  the  entire 
data  for  this  period.  For  the  remaining  period  of  selection  with  this 
line,  10j^  months,  the  mean  reaction-times  were  375.4  seconds  and 
400.7  seconds  for  the  plus  and  minus  strains,  respectively.  The  dif- 
ference was  —25. 3 ±13. 46  seconds,  a  difference  not  of  statistical  sig- 
nificance. There  was  again  a  considerable  number  of  same-day  broods 
(13),  for  which  the  mean  reaction-times  were  350  and  345  seconds  and 
the  difference  +5  seconds. 

A  test  series  conducted  in  January  1914,  consisting  of  328  indi- 
viduals in  the  plus  strain  and  327  individuals  in  the  minus  strain,  gave 
a  mean  reaction-time  of  416.1  seconds  for  the  plus  strain  and  433.9 
seconds  for  the  minus  strain.  The  difference  was  —17.8 ±13. 9 
seconds.  This  difference  like  the  others  is  not  of  statistical  value. 

A  curve  for  the  reaction-time 
means  of  this  line  is  shown  in  figure 
9  and,  like  the  numerical  summaries 
just  considered,  shows  that  there  is 
probably  no  effect  of  selection. 
There  is  just  a  possible  suggestion 
of  an  effect  of  selection,  however,  in 
the  last  year's  data,  inasmuch  as 
during  this  period  the  plus  strain 
was  continuously  the  more  reactive, 
FIGURE  9.— Line  751.  except  for  one  two-month  period. 

Reaction-time  curves.  -r.    j    •          •  f  ji  j_  n       ,        ,• 

But  in  view  of  the  great  fluctuation 

in  the  reaction-time  curves  for  the  other  lines  subjected  to  selection, 
this  suggestion  has  little  weight. 

There  is  a  feature  of  interest  in  the  curves  for  this  line,  as  with 
many  other  lines,  the  general  close  correspondence  in  the  levels  of 
the  plus  and  minus  curves  in  the  different  two-month  periods;  on 
the  whole  they  sweep  upward  and  downward  together,  an  expression 
of  a  parallel  effect  of  environment  upon  the  two  strains. 

It  is  a  matter  of  interest  and  some  importance  to  note  that  the 
means  for  the  two  strains  of  Line  751  start  at  approximately  the 
same  levels  which  were  held  by  the  other  D.  pulex  selection  lines 
(Lines  689,  691,  695,  711,  713,  714,  and  719)  during  the  same  two- 
month  period,  although  the  other  lines  had  been  subjected  to  labora- 
tory culture  for  approximately  a  year  and  to  selection  for  8  months 
before  Line  751  was  brought  into  the  laboratory.  Comparison  of  the 
curves  for  Line  751  (figure  9)  and  for  the  other  D.  pulex  lines  (figures 
2c,  3c,  4,  5,  6,  7c,  and  8c)  and  with  the  composite  curves  for  all  the 
D.  pulex  plus  and  minus  strains  (figure  10D,  on  which  the  curves  for 
Line  751  are  superimposed),  strikingly  brings  out  this  fact.  This 
is  important  as  showing  that  under  laboratory  conditions  strains  of 


8-9 


1913 


68 


SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 


a  given  species  tend  to  have  approximately  the  same  reaction-time 
means,  regardless  of  the  length  of  their  previous  laboratory  history. 

LINE  762. 

Line  762  was  one  of  the  two  lineally  distinct  lines  of  D.  longispina 
subjected  to  selection.  It  was  obtained  October  19, 1912,  from  Pond 
I,  one  of  the  surface-water  ponds  on  the  upland,  and  was  subjected 
to  selection  at  once.  This  is  a  less  favorable  species  for  selection  on 
the  basis  of  reactiveness  to  light  than  the  other  species  used.  While 
it  is  moderately  reactive,  it  does  not  so  readily  withstand  the  hand- 
ling, and  individuals  sometimes  become  disabled  or  become  lodged 
in  the  surface  film  of  the  water. 

The  summary  of  the  data  for  Line  762  is  found  in  table  29  and 
in  figure  10A.  The  experiment  with  this  line  was  of  relatively  short 
duration  (10  months)  as  compared  with  the  other  lines,  due  to  the 

TABLE  29. — Selection  summary  for  Line  762. 


Time  period. 


Nov.  1912- 
July  31,  1913 

Aug.  1,  1913- 
Sept.  8,  1913 


Strain. 


Plus.  . 
Minus 


Minus  .  . 


1-30 

1-28 
31-36 

29-35 


•i  . 


6  "8 


Ave 
per 


7  11 


6.9 
6.4 

9.8 
.3 


1 


03 

ii 


215 

/S5 

59 

79 


No.  of  net 
dividual? 


3  o 


168 


111 

147 


Aver 
tim 


573 
(505 
581 

500 


366.2 
377.3 
269.3 

455.3 


229.2 
257.1 
224.1 
305. 5 


10.5 


19.7 

23.2 


8 1 

g  « 
|-2 

gS 


-//.i 

-2/s.o 


oba 


16.19 
36!4jg7.00| 


iffe 
ble 


accidental  loss  of  the  plus  strain  in  September  1913.  Shortly  after 
this  loss  the  experiment  was  resumed  by  selecting  in  both  directions 
from  the  minus  strain.  The  line  was  henceforth  designated  Line 
766.  This  was  in  effect  a  return  selection  within  the  minus  strain.1 

The  data  for  the  first  nine-month  period  (November  1912- 
July  1913,  table  29)  gives  reaction-time  means  of  366.2  and  377.3 
seconds.  The  difference  (-11.1  d=  16.19  seconds)  is  not  significant. 
The  10  same-day  broods  for  this  line,  comprised  mostly  of  broods 
produced  during  June  and  July,  gave  the  following  means:  355 
seconds  for  the  plus  strain  (57  individuals)  and  349  seconds  for  the 
minus  strain  (54  individuals).  The  same-day-brood  data  rather 
clearly  substantiate  the  conclusion  which  the  curves  (figure  10 A) 
suggest,  that  until  that  time  there  was  no  effect  of  selection. 

For  the  final  portion  of  the  experiment,  a  little  more  than  a 
month  (until  the  plus  strain  was  lost),  the  plus  strain  was  markedly 

1  It  is  unfortunate  that  return  selections  were  not  conducted  in  other  lines,  particularly  in 
Line  757.  The  return  selection  in  Line  762  (which  then  became  Line  766)  is  not  significant  as  a 
return  selection,  because  presumably  there  was  not  an  effect  of  selection  within  Line  762. 


3( 


A   PHYSIOLOGICAL   CHARACTER. 


69 


the  more  reactive,  the  means  being  269  and  482  seconds  and  the 
difference  -213  ±  30.42  seconds.  There  is,  moreover,  an  interesting 
difference  noted  between  every  plus  brood  and  every  minus  brood 
tested  during  this  period.  Without  exception  the  plus  broods  were 
considerably  more  reactive  than  the  minus  broods  tested  at  the 
nearest  dates.  Such  a  consistent  difference  in  reactiveness  involving 
every  individual  brood1  is  not  generally  found  in  the  data  obtained 
from  these  selection  experiments,  except  in  portions  of  the  data  for 
Line  757,  in  which  a  pronounced  effect  of  selection  was  obtained. 


750  r 


600 


450 


300 


12-1 


1913 

4-5        8-9 


12-1 


1914 
4-5        8-9 


12-1 


\ 


1915 
4-5       8-9 


12-1 


1916 
4-5          8 


4-5        8-9 
1912 


12-1 


4-5        8-9 
1913 


12-1 


4-5       8-9 
1914 


12-1 


4-5       8-9 
1915 


12-1 


4-5       8-9 
1916 


FIGURE  10. 

A,  B,  C.  Reaction-time  curves  for  Daphnia  longispina — Lines  762, 766,  and  768, respectively. 
D.  Composite  reaction-time  curves  for  all  plus  and  all  minus  strains  of  Daphnia  pulex 
with  reaction-time  curves  for  Line  751  superimposed. 

The  divergence  in  reaction-time  means  for  this  last  two-month 
period  and  particularly  the  complete  and  unusual  consistency  of 
differences  in  reaction-time,  brood  by  brood,  for  this  last  period 
suggest  an  effect  of  selection.  The  amount  of  these  data  is  too  small 
to  be  more  than  slightly  suggestive,  but  it  seems  quite  possible  that 
a  mutation  occurred  in  the  plus  strain  of  Line  762,  thus  producing 

1lt  will  be  remembered  that  temporary  enviromental  conditions  are  very  potent  factors 
in  modifying  the  reaction-time  means  of  individual  broods  and  that  consequently  rather  wide 
fluctuations  occur  in  all  the  data.  Hence,  only  in  cases  in  which  the  reactiveness  of  the  two 
strains  differs  greatly  do  we  fail  to  find  the  reaction-time  means  for.  different  broods  of  the  two 
strains  of  a  line  overlapping  to  a  wide  degree. 


70 


SELECTION    IN    CLADOCERA    ON   THE    BASIS    OF 


the  great  lowering  of  the  plus  mean  and  the  marked  differences  in 
reactiveness  between  the  two  strains  not  present  earlier. 

LINE  766. 

As  noted  above,  the  two  strains  of  this  line  were  derived  from 
the  minus  strain  of  Line  762.  The  minus  strain  was  a  continuation 
of  the  minus  strain  of  Line  762,  the  selection  having  been  inter- 
rupted for  three  generations.  The  plus  strain  of  Line  766  was  de- 
rived from  the  minus  strain  of  Line  762  by  selecting  the  most  reactive 
individual  from  the  thirty-ninth  generation  of  the  minus  strain  of  that 
line.  The  summary  of  the  data  is  given  in  table  30  and  figure  10s. 
Selection  was  continued  for  only  7  months,  when  it  was  interrupted 
by  the  loss  of  the  plus  strain. 

A  rather  wide  difference  in  mean  reaction-times  for  the  first 
two-month  period  is  not  continued  during  the  remainder  of  the  ex- 


TABLE  30.  —  Selection  summary  for  Line  766. 


Time  period. 


Oct.     1,     1913- 
Apr.  1914 


Strain. 


Plus .  . . . 
Minus .  . 


39_61 
40-56 


18 


7.0 
6.4 


i 


154 
116 


10 


182 

822 


622 
695 


327.0 
385. 8 


247.3 
268.8 


13.4 
16.8 


« .5 
§t 


Di 
ac 


-58.8 


ffere 
le 


.7 


periment.  The  mean  reaction-times  for  the  two  strains  for  the 
entire  period  of  the  experiment  were  327  and  385.8  seconds  (table 
30).  While  the  difference  is  —58.8  seconds  and  is  2.72  times  the 
probable  error,  it  is  largely  due  to  the  difference  in  mean-reaction 
time  obtaining  for  the  first  two  months.  There  were  two  same-day 
broods  containing  totals  of  only  11  and  9  individuals  in  the  plus  and 
minus  strains,  respectively.  The  averages  were  222  and  250  seconds 
and  the  difference  was  —  28  seconds,  a  difference  which,  with  so  few 
individuals,  is  not  significant.  There  is  no  effect  of  selection. 

The  effect  of  environmental  influences  upon  reaction-time  is 
indicated  by  the  generally  coincident  up-and-down  courses  of  the 
curves  of  the  two  strains  of  Line  762  and  Line  766  (see  figure  10, 
A  and  B). 

LINE  768. 

Line  768  was  the  second  of  the  two  genetically  distinct  lines  of 
D.  longispina.  It  was  taken  into  the  laboratory  from  Pond  IV  in 
November  1913,  but  selection  was  not  begun  until  December  1914 


A   PHYSIOLOGICAL   CHARACTER. 


71 


in  the  fifty-fourth  laboratory  generation.  Selections  were  continued 
for  21  months,  75  and  74  generations  in  the  plus  and  minus  strains, 
respectively.  The  data  will  be  found  in  tables  31  and  32  and  in 
figure  lOc. 

The  data  for  the  first  longer  period  (December  1914-July  1915) 
consists  of  29  and  28  broods  from  the  two  strains,  there  being  238 
and  239  individuals.  The  mean  reaction-times  were  426  and  443 
seconds.  The  difference  is  -17 ±19.5  seconds  (table  31).  There 

TABLE  31. — Selection  summary  for  Line  768. 


Time  period. 


Dec.  21,  1914- 
July  31,  1915 

Aug.  1,  1915- 
July  31,  1916 

Aug.  1,  1916- 
Aug.  22,  1916 


Strain. 


Plus .... 

Minus  . . 
Plus. ... 
Minus  . . 
Plus.... 

Minus  . . 


54-83 

64-82 

84-125  39 

88-124  38 
126-128 
125-127 


11 


"s 

0*0 

55  - 


8.2 
8.5 
9.7 
7.7 
9.7 
7.3 


|| 
1  I 


238 
239 
376 

29 


f. 
II 


"S- 


211 
181 
111 

143 

65 


730 

657 
647 
845 
507 


426.0 
443.0 
315.7 
328.0 
350.0 
155.4 


331.2 
299.7 
242.1 
2S0.5 
268.5 
186.1 


14.5 

8.'4 

S.I 

33.6 

26.8 


ss 

Q  rt 


-17.0 


-12. S 
+151.6 


43. 


.50.57 


3.52 


TABLE  32. — Same-day  broods.    Summary  of  data  for  Line  768. 


M 

ft 

tj-g 

£ 

a 

3 

k 

h 

Time  period. 

Strain. 

| 

3  No.  of  yo 
ood. 

ndividuals. 

negatively 
individuals 

ndividuals 
reach  an 
tank. 

3  minimum 
-time. 

3  maxim 
>n-time. 

individual 
-time. 

nee  betw 
reaction-tin 

0 
0 

"3 
6 

a 

0    « 

°  *^ 

II 

|| 

i! 

®  s 

m  g 

55 

<j  u 

55 

55  * 

fc 

«!* 

<* 

S  * 

S 

Dec.  21,  1914-July  31, 

plus.... 

17 

10.6 

180 

2 

52 

212 

808 

454 

+  16 

1915  

. 

Aug.     1,  1915-July  31, 

f  Plus.... 

4 

12.5 

50 

1 

3 

106 

603 

374 

+  38 

1916  

1  ,,  . 

/ 

9  5 

S9 

o 

# 

118 

615 

S36 

were  a  large  number  (17)  of  same-day  broods  for  this  period,  for 
which  the  mean  reaction-times  were  454  and  438  seconds.  The  dif- 
ference is  +16  seconds  (table  32). 

For  the  year-period  (August  1915-July  1916)  there  were  39 
and  38  broods  consisting  of  376  and  294  individuals  in  the  two 
strains.  The  mean  reaction-times  were  315.7  and  328  seconds.  The 
difference  is  -12.3  ±12.4  seconds.  There  were  only  4  same-day 
broods,  for  which  the  mean  reaction-times  were  374  and  336  seconds. 
The  difference  is  +38  seconds. 


72  SELECTION    IN    CLADOCERA    ON    THE    BASIS    OF 

This  experiment  was  continued  during  the  most  of  another 
month,  but  the  data  consists  of  the  records  for  only  3  broods  from 
each  strain.  The  means  are  350  and  198.4  seconds.  The  difference 
(  +  151.6  seconds),  while  large,  is  based  upon  entirely  too  few 
individual  reaction-times  to  be  assigned  significance.  It  is  obvious 
that  with  Line  768  there  was  no  effect  of  selection. 

The  reproductive  indices  for  this  line  have  not  been  worked  out, 
but  the  plus  strain  was  apparently  somewhat  the  more  vigorous. 
That  this  difference  in  vigor  has  not  influenced  reaction-time,  to  any 
considerable  extent  at  any  rate,  is  evident  from  the  fact  that  the 
same-day  broods  of  the  plus  strain  were  on  the  whole  less  reactive 
than  those  of  the  minus  strain. 

There  is  again,  as  with  many  other  lines,  an  obvious  effect  of 
environmental  conditions  upon  reaction-time.  In  spite  of  rather 
wide  local  fluctuations,  the  means  for  the  two  strains,  on  the  whole, 
follow  each  other  rather  closely,  as  is  shown  in  figure  lOc. 

Reference  to  figure  10  (in  which  are  given,  separately,  the  re- 
action-time curves  for  the  three  selection  lines  of  D.  longispina  and 
composite  curves  for  the  reaction-times  of  all  the  D.  pulex  selection 
lines)  shows  that  there  is  a  striking,  though  somewhat  rough,  paral- 
lelism between  the  reaction-time  curves  for  the  D.  longispina  and 
the  D.  pulex  lines.  This  is  an  expression  of  similar  influences  of  the 
same  environmental  factors  upon  the  reactiveness  to  light  of  the 
two  species. 


A   PHYSIOLOGICAL   CHARACTER.  73 

GENERAL  INTRODUCTION  FOR  SIMOCEPHALUS 
EXSPINOSUS  LINES. 

S.  exspinosus  in  some  regards  seemed  an  unfavorable  species  for 
selection  on  the  basis  of  its  reaction  to  light.  During  the  tests  indi- 
viduals frequently  attached  to  the  surface  film  or  to  the  sides  of 
the  experimental  tank  and  rested  there  during  the  remainder  of  the 
test  (see  also  page  25).  Further,  and  to  an  even  greater  degree,  the  S. 
exspinosus  young  were  unsatisfactory  in  that  they  so  generally  settled 
to  the  bottom  of  the  tank  and  appeared  non-reactive  to  light.  In 
most  broods  there  were  several  individuals  which  failed  to  reach 
either  end  of  the  tank  during  the  15  minutes  of  the  experiment,  while 
in  some  broods  (particularly  during  the  earlier  course  of  the  experi- 
ments) there  was  no  response  on  the  part  of  any  individual  of  the 
brood.  The  reactiveness  to  light  during  the  early  course  of  the  ex- 
periments was  so  slight  that  in  many  cases  there  were  really  no 
grounds  for  making  a  selection  and  there  seemed  little  hope  of  a 
sufficient  reactiveness  to  light  to  afford  a  basis  for  conducting  an 
experiment  in  selection  on  this  character. 

Several  of  the  earlier  S.  exspinosus  lines  were  discarded  as  un- 
profitable after  a  few  generations  of  selection.  Lines  740  and  757, 
while  not  promising  at  the  start,  were  continued  and  subjected  to 
selection  for  a  time  to  see  if  it  then  seemed  advisable  to  continue  the 
selections.  Even  comparatively  slightly  reactive  material  might 
conceivably  afford  a  basis  for  selection  if  there  were  enough  reactive 
individuals  to  make  a  selection  possible  in  a  considerable  percentage 
of  cases.  In  addition  to  broods  (particularly  the  earlier  broods  of 
these  two  lines)  which  showed  absolutely  no  reaction  to  the  light, 
there  were  other  broods  in  which  the  only  selection  possible  (in  the 
plus  strain)  was  from  among  individuals  which  had  moved  very 
slightly  toward  the  light.  In  the  minus  strains  the  only  choice  in  a 
vast  majority  of  cases  was  an  individual  which  showed  no  reaction  to 
light.  There  were  numbers  of  these  in  nearly  every  brood.  Negatively 
reacting  individuals  were  so  rare  with  this  species  that  it  was  seldom 
possible  to  make  selections  in  the  minus  strains  on  the  ground  of  a 
negative  reaction  to  light1.  But  after  a  few  generations  it  was  con- 
sidered that  the  reactiveness  of  the  strains  of  Lines  740  and  757 
seemed  sufficient  to  justify  the  continuation  of  these  lines  in  the  hope 
that  there  was  sufficient  reactiveness  to  test  out  the  possibility  of 
selection.  The  results  justify  the  conclusion  then  reached,  that  after 
all  there  was  sufficient  basis  for  an  experiment  in  selection. 

It  is  worthy  of  emphasis  that  the  selections  in  the  minus  strains 
of  lines  of  Simocephalus  were  rarely  on  the  ground  of  a  negative 

1  However,  as  noted  elsewhere  (page  16),  it  is  questionable  if  there  is  any  real  significance  to 
be  assigned  to  the  negative  reactions  of  the  few  individuals  which  went  to  the  negative  end  of 
the  experimental  tank. 


74 


SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 


reaction  to  light;  that  the  only  selection  possible  in  very  many  cases 
was  from  among  individuals  showing  no  reaction;  and  that  in  the 
remaining  cases  the  selections  in  the  minus  strains  were  postively 
reacting  individuals  with  the  highest  reaction-times. 

Lines  740  and  757  were  subjected  to  selection  from  August  and 
November  1912  to  May  1917.  Lines  794,  795,  and  796  were  used  in 
selection  from  December  1914  to  May  1917.  All  these  lines  of  S. 
expinosus,  except  Line  757,  originated  from  mothers  obtained  from 
Pond  IV,  the  larger  surface-water  pond  on  the  hill,  a  mile  from  the 
laboratory  of  the  Station  for  Experimental  Evolution. 

TABLE  33. — Selection  summary  for  Line  794- 


Time  period. 


Dec.  23.  1914- 

July  31,  1915 
Aug.  1,  1915- 

July  31,  1916 
Test  series, 

July  1916... 
June  1,  1916- 

Aug.  31,  1916 
Aug.  1,  1916- 

May  1,  1917 


Strain. 


Plus.. 

Minus 
Plus.  . 
Minus 
Plus .  . 
Minus . 
Plus .  . 
Minus 
Plus.. 
Minus 


o 


1-27 

1-26 

28-63 

27-61 

62 

61 

59-67 
69-64 
64-91 
62-90 


00 
55-0 


13.3 

26  11.5 
15.0 
32  14.7 
.5 

36  26.8 
15.6 
5.7 
15.1 


3627. 
'6 

7 

e 

24 


23  12.9 


358 

495 
471 
989 
565 
109 

363 


It 
P 


I! 


„ 


115 

65 
171 

239 

254 

16 

64 

49 


218 

195 
316 
102 

53 
104 

53 
163 
182 


829 

738 
804 

647 
654 
720 

625 
535 


529.4 
451 .8 
483.8 
555.1 
395.9 
355. 2 
314.4 
409.6 
309.9 
373. 8 


301.0 
284.4 
325.2 
334.3 
309.4 
324.9 
266.4 
325.5 
290.4 
289.9 


10.7 

11.3 

9.9 

10.4 

6.6 

7.1 

17.2 

29.2 

10.3 

11.3 


+  77.6 


15  s 

+  .7 


-63.  9 


15.6 


9.7 
33!  5 


-° 


4.97 


TABLE  34. — Same-day  broods.    Summary  of  data  for  Line  794- 


I 

.9 

5 

i 

i 

1 

i 

•2 

g 

A 

I 

i 
1 

li 

i 

0 

$ 

1.1 

S'g 

i 

ii 

c. 

Time  period. 

Strain. 

No.  of  broods. 

1 

No.  of  individuals. 

No.  of  negatively  r< 
dividuals. 

No.  of  individuals 
reach  an  end  of  th 

Average  minimum 
time. 

Average  maximum 
time. 

Mean  individual 
time. 

Standard  deviation 

i 

Difference  between 
plus  and  minus  st 

Probable  error  of 

Difference  divided 
ble  error. 

Dec.  23,  1914- 

{  Plus.. 

7 

13  1 

92 

1 

22 

155 

876 

475 

276.95 

19.48 

+  116 

25.78 

4.49 

July  31,  1915 

\  Minus  .  . 

7 

11.6 

81 

8 

173 

681 

355 

z!25  .  50 

16.90 



Aug     1    1915 

(Plus 

r 

16  2 

81 

o 

28 

259 

900 

463 

July  31    1916 

r, 

IS  8 

69 

10 

276 

!K)i> 

490 

—  27 

Aug      1    1916— 

(  Plus 

•> 

12  0 

24 

o 

4 

105 

645 

323 

270  21 

37  20 

May    1,  1917 

\  Minus  '.  '. 

i 

13.0 

26 

0 

4 

816 

645 

457 

248.69 

32.90 

-134 

49.66 

2.69 

LINE  794. 

The  selection  data  for  Line  794  are  given  in  tables  33  and  34 
and  figure  11. 

This  line  and  Lines  795  and  796  originated  from  three  mothers 
collected  December  11,  1914.  Selection  was  begun  with  the  first 


A   PHYSIOLOGICAL   CHARACTER.  75 

broods  of  young  produced  in  the  laboratory  and  continued  for  29 
months,  91  generations  in  the  plus  strain  and  90  generations  in  the 
minus  strain. 

For  the  first  longer  period  (December  1914- July  1915)  of  the 
experiment  the  means  for  the  plus  and  minus  strains  (358  and  288 
individuals)  were,  respectively,  529.4  and  451.8  seconds  (table  33). 
The  difference  (+77.6dbl5.6  seconds)  was  4.97  times  the  probable 
error.  For  the  first  single  month  of  selection  (December  1914)  the 
plus  strain  was  more  reactive  by  40  seconds  and  again  in  June  1915 
the  plus  strain  was  the  more  reactive  strain,  this  time  by  a  large 
margin,  274  seconds;  but  for  the  other  6  months  of  the  first  longer 
period  the  minus  was  more  reactive  by  differences  ranging  from  88 
to  219  seconds.  For  the  same-day  broods  (table  34)  the  means 
were  475  and  359  seconds,  the  plus  strain  having  a  higher  mean  by 
116  ±  25.78  seconds.  Of  the  7  same-day  broods  for  this  period,  5 
occurred  in  succession  in  January  and  February  1915,  almost  im- 
mediately after  the  beginning  of  the  experiment.  The  differences 
were  +175,  +207,  +193,  +140,  and  +152  seconds.  Data  for  a 
pair  of  same-day  broods  occurring  just  before  these  five  are  incomplete, 
but  the  minus  brood  was  the  more  reactive  by  at  least  196  seconds, 
while  the  pair  of  broods  just  preceding  those  last  mentioned  were 
likewise  same-day  broods  with  the  plus  the  more  reactive  by  a  small 
margin  (27  seconds).  Thus  there  were  in  effect  7  successive  same-day 
broods,  in  all  of  which  the  minus  brood  was  considerably  the  more 
reactive.  These  differences  are  so  large  and  so  persistent  as  to  indi- 
cate for  this  early  and  limited  period  (January— February  1915)  a 
real  difference  in  reaction-time,  with  the  minus  strain  the  more 
reactive.  In  the  last  2  months  of  this  longer  period  (to  August  1915) 
the  minus  strain  had  much  the  larger  reaction-time,  and  this  differ- 
ence was  greatly  increased  in  the  next  two-month  period  and  con- 
tinued very  large  for  another  two-month  period.1 

The  second  longer  period  (the  year  August  1915- July  1916) 
gave  averages  of  483.8  and  589.1  seconds.  The  difference  was 
-105.3  ±14.3  seconds,  or  7.36  times  the  probable  error.  For  the 
same  period  the  5  same-day  broods  had  a  difference  of  —27  seconds. 
This  year's  data,  taken  as  a  whole,  might  at  first  thought  be  con- 
sidered suggestive  of  an  effect  of  selection;  but  the  great  irregularities 
[of  the  curves,  the  small  difference  between  same-day-brood  means, 
and  the  large  differences  in  the  opposite  direction  for  the  preceding 
i>year  do  not  favor  such  an  interpretation.  A  test  series  conducted 
[during  July  1916,  consisting  of  nearly  1,000  individuals  of  each 
strain,  gave  means  differing  by  only  +0.7  ±9.7  seconds.  This  test 

1  The  very  great  divergence  in  the  two  reaction-time  curves,  the  minus  strain  being  the 
reactive  of  the  two,  for  the  six-month  period  June  to  November  1915  is  no  more  easily 
>unted  for  than  the  earlier  six-month  period,  during  which  the  plus  strain  was  markedly  the 
reactive. 


76  SELECTION   IN    CLADOCERA   ON   THE    BASIS   OF 

series  shows  effectively  that  there  was  up  to  July  1916  no  certain 
basis  for  assuming  an  effect  of  selection  within  this  line. 

During  the  last  longer  period  (August  1916-May  1917)  of 
selection  with  Line  794  the  means  were  309.9  and  373.8  seconds.  The 
difference  (—  63. 9  ±15. 3  seconds)  was  4.18  times  its  probable  error. 
The  2  same-day  broods  gave  means  differing  by  —134  ±49. 66 
seconds.  As  for  the  preceding  year,  the  means  are  suggestive  of  an 
effect  of  selection,  but  the  irregular  course  of  the  curves  (figure  lie), 
together  with  the  result  of  the  test  series  conducted  in  July  1916 
(table  33)  and  the  much  higher  reaction-time  for  the  plus  strain 
during  6  months  of  the  experiment,  make  this  interpretation  ques- 
tionable. It  is  noteworthy,  however,  that  in  spite  of  considerable 
irregularities  in  the  curves,  the  curve  for  the  minus  strain  is  appreci- 
ably lower  than  that  for  the  plus  strain  in  only  one  of  the  9  two- 
month  periods  after  the  first  12  months  of  the  experiment.  After 
this  early  period  all  the  data  except  the  test-series  data  indicate  that 
the  minus  strain  was  on  the  whole  the  less  reactive  throughout  the 
experiment. 

Since  the  mothers  from  which  the  two  strains  of  Line  794 
originated  were  sisters  from  the  same  brood,  it  is  hard  to  understand  to 
what  the  difference  in  reactiveness  (the  minus  strain  being  markedly 
the  more  reactive)  during  the  period  of  the  experiment  (June- 
November  1915)  can  have  been  due.  It  is  possible  to  think  of  it 
as  a  mutation  in  the  minus  strain,  the  effect  of  which  was  later 
eliminated  by  selection  or  by  a  second  mutation  in  the  same  strain 
in  the  direction  of  less  reactiveness  to  light;  but  these  are  mere  sur- 
mises for  which  there  is  no  real  evidence  and  the  differences  them- 
selves are  not  pronounced  and  certain  enough  to  be  considered  as  of 
great  significance.  It  is  probable,  however,  that  the  rather  wide 
differences  in  reactiveness  in  the  minus  strain  of  this  line  at  different 
periods  of  the  experiment  are  due  to  non-genetic  factors,  such  as 
produced  similar  effects  in  Lines  695  and  740  (see  figures  2c  and  15) 
and  elsewhere.  Nevertheless,  the  data  leave  room  for  the  suggestion 
that  selection  was  possibly  responsible  for  the  elimination  of  the 
greater  reactiveness  on  the  part  of  the  minus  strain,  however  it 
may  have  occurred,  and  a  development  of  a  relatively  greater  re- 
activeness on  the  part  of  the  plus  strain. 

Figure  HA  indicates  graphically  the  general  reproductive  levels 
for  the  two  strains  during  the  different  two-month  periods.  There 
is  no  obvious  relation  between  reproductive  vigor  and  mean  reaction- 
time  unless  the  generally  lower  reproductive  vigor  of  the  minus 
strain  be  assumed  to  account  for  the  generally  higher  reaction-time 
of  the  minus  as  compared  with  the  plus  strain;  but  with  Line  795 
(see  figure  12s)  the  plus  strain  in  general  showed  an  even  greater 
superiority  in  vigor,  yet  it  was  much  less  reactive  than  the  minus 


A   PHYSIOLOGICAL   CHARACTER. 


77 


strain  of  the  same  line.    Examined  in  detail,  the  data  for  reproductive 
vigor  and  reaction-time  do  not  appear  to  be  related.1 


2.50r 


2.00 


1.50 


1.00 


.50 


00 


12-1         4-5        8-9       12-1         4-5        8-9       12-1          4 
1915  1916  1917 


750 


60O 


ABO 


300 


150 


T_T 


B       1 


; 


12-1 


4-5        8-9 
1316 


12-1          4 
1917 


1.00 
.75 
.50 
.25 
00 
.25 
.50 


FIGURE  11. — Line  794. 

A.  Reproductive  indices,  actual  values. 

B.  Reproductive  indices,  superiority. 

C.  Reaction-time  curves. 

LINE  795. 

Line  795  is  a  sister  line  of  Lines  794  and  796  and  was  subjected 
to  selection  for  a  like  period  covering  94  generations  for  the  plus 
and  95  generations  for  the  minus  strain.  The  data  will  be  found  in 
tables  35  and  36  and  in  figure  12. 

1  For  example,  during  the  first  6  months  the  plus  strain  of  Line  794  was  persistently  the 
more  vigorous  and  generally  the  less  reactive.  The  7  high  points  in  figure  HA  showing  high 
reproductive  indices  (above  1.75)  for  the  plus  strain  correspond  4  times  with  periods  of  greater 
reactiveness  in  the  same  strain,  and  3  times  with  periods  of  less  reactiveness;  5  similar  high  points 
of  reproductive  indices  for  the  minus  strain  correspond  with  1  period  of  relatively  greater  re- 
activeness in  the  minus  strain  and  with  4  points  of  lesser  reactiveness.  The  two-month  periods 
of  relatively  low  reproductive  indices  for  the  two  strains  likewise  fail  to  correspond  with  periods 
of  lesser  reactiveness  to  light. 


78 


SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 


Casual  examination  seems  to  reveal  at  once  the  fact  that  there 
is  no  effect  of  selection,  and  this  is  probably  correct,  but  on  careful 
analysis  it  is  not  altogether  certain  that  an  effect  of  selection  is 
entirely  lacking.  The  plus  strain  is  generally  the  less  reactive,  but 
the  curve  for  the  minus  strain  is  extremely  fluctuating. 

For  the  first  longer  period  (8  months)  the  mean  for  the  plus 
strain  was  703.3  seconds,  for  the  minus  strain  571.4  seconds  (table 

TABLE  35. — Selection  summary  for  Line  795. 


Time  period. 


Dec.  24,  1914- 

July  31,  1915 
Aug.  1,  1915- 

July  31,  1916 
Test  series, 

June  1916... 
May  1,  1916- 

July  31,  1916 
Aug.  1,  1916- 

May  1,  1917 


Strain. 


Plus.. 

Minus 
Plus.  . 
Minus 
Plus.. 
Minus 
Plus.. 
Minus 
Plus.  . 
Minus 


1-27 

1-27 

28-65 

28-64 

60 

60 

57-65 
56-64 
66-94 
65-96 


24 


10. 
2263. 


S£  60. 8  1338 


16. 


1 

8  10.3 
8 

4 


14. 


20  14. 


3  £ 
£"« 

"H  £ 


292 
.£«? 
580 
376 
1397 


129 


356 

2S7 


2 

i! 


164 
103 
301 

733 

683 

60 

115 

02 


425 
317 
325 

230 
188 
181 

133 


900 
875 
842 
70S 
880 
SS3 
750 
776 
815 
735 


703.3 
671.4 
625.0 
530.7 
615.4 
591.6 
594.7 
44S.1 
457.2 
502.0 


254.4 

500.  i 

308.5 
323.1 
330.9 
366.1 
316.7 
341.7 
326.4 
330.1 


10.0 

12.1 

8.6 

11. £ 

6.0 

6.6 

18.8 

25.5 

11.7 


II 


+  131.9 
'+94 '.3 


+23.8 


+  146.6 
"-44.8 


15.7 


8.40 


14.26.64 
'  8!9  2. 67 


31.64.63 

17.62.66 


TABLE  36. — Same-day  broods.    Summary  of  data  for  Line  795. 


Time  period. 


Dec.  24,  1914- 
July  31,  1915 

Aug.  1,  1915- 
July  31.  1916 

Aug.  1,  1916- 
May  1,  1917 


Strain. 


Plus... 
Minus  . 
Plus... 
Minus  . 
Phis... 
Minua  . 


1210. 


12.6 


6  14- 


139 

162 

1*3 

63 

73 


"8 


363 
354 
334 
278 
117 


900 
S7S 
725 
818 
714 
000 


661 
50S 
617 
470 
307 
671 


i 


277.77 
299.17 
323.84 
337.18 
263.21 
SIS. 97 


15.89 
17. S4 
17.16 
20.51 
22.37 
£4.78 


1| 

cl 

4J     0 

IS 
1.2 

•S  a 

I -a 


+63 

+'i38' 


23.89 
26!  74 


S3.3S 


2.63 


35).  The  difference  was  +131. 9 ±15. 7  seconds,  or  8.4  times  the 
probable  error.  The  12  same-day  broods  for  this  period  likewise  gave 
a  mean  for  the  plus  strain  in  excess  of  the  minus  strain  (table  36). 
The  difference  was  -f63  ±23.89  seconds,  2. 63  times  the  probable  error. 
For  the  second  longer  period  (August  1915 — July  1916)  of  the 
data  for  this  line  the  means  were  625.0  and  530.7  seconds  for  the 
plus  and  minus  strains.  The  difference  was  4- 94. 3  ±14. 2  seconds. 
This  difference  was  6.64  times  the  probable  error.  Twelve  same- 


A   PHYSIOLOGICAL   CHARACTER. 


79 


day  broods  averaged  617  and  479  seconds,  the  difference  being 
+  138=1=26.74  seconds.  A  test  series  conducted  in  June  1916,  and 
consisting  of  more  than  1,300  individuals  of  each  strain,  gave  a  plus 
mean  of  615.4  seconds  and  a  minus  mean  of  591.6  seconds.  The 
difference  (+23. 8  ±8. 9  seconds)  was  2.67  times  the  probable  error. 
Thus  the  data  for  this  line  to  August  1916  indicates  a  signifi- 


2.00 
1.50 
1.00- 
.50 


OO 


l- 


12-1          4-5         8-9        12-1         4-5         8-9        12-1  4 

1915  1916  '917 


750 


600 


300 


I50H 


f> 


12-1 


4-5         8-9 
1915 


12-1 


4-5 


12-1 


1916 


/ 


4 
1917     - 


FIGURE  12. — Line  795. 

A.  Reproductive  indices,  actual  values. 

B.  Reproductive  indices,  superiority. 

C.  Reaction-time  curves. 

cantly  lower  reactiveness  on  the  part  of  the  plus  strain,  a  result  op- 
posite that  of  a  selective  effect. 

For  the  following  and  final  9  months  of  selection  with  this  line 
the  plus  strain  averaged  the  more  reactive;  the  means  were  457.2 
and  502  seconds,  and  the  difference  was  —44. 8  ±17. 6  seconds,  or 
2.55  times  the  probable  error.  The  5  same-day  broods  gave  as  means 
307  and  671  seconds.  The  minus  same-day  mean  was  more  than 
double  that  for  the  plus  strain,  the  difference  being  —364  =t  33.38 
seconds,  10.9  times  the  probable  error.  Of  these  5  same-day  broods, 


80  SELECTION   IN   CLADOCERA   ON   THE   BASIS   OF 

4  occurred  in  succession  in  April  1917  and  constituted  the  final  data 
for  this  line.  The  writer  believes  that  considerable  stress  may  be 
safely  placed  upon  the  same-day-brood  data  and  is  inclined  to  think 
that  at  the  close  of  the  experiment  with  Line  795  the  plus  strain  was 
actually  and  significantly  the  more  reactive. 

It  is  quite  possible  to  suppose  that  the  considerably  higher 
reaction-time  of  the  plus  strain  during  the  early  part  of  this  experi- 
ment (whatever  may  have  been  the  cause  and  meaning)  was  gradually 
reduced  by  selection  and  that  at  the  close  of  the  experiment  the  plus 
strain  was  actually  the  more  reactive,  due  to  the  influence  of  selec- 
tion. But  in  the  absence  of  a  test  series  at  the  close  of  the  experiment, 
and  without  knowledge  as  to  what  the  relative  reaction-time  means 
for  the  two  strains  would  have  been  in  later  generations,  with  or 
without  further  selection,  the  supposition  is  not  worth  consideration. 

It  is  not  at  all  improbable,  however,  that  the  differences  in 
different  parts  of  the  curves  for  the  two  strains  represent  merely  low 
and  high  points  in  the  reactiveness  of  the  two  strains  due  to  non- 
genetic  influences,  the  plus  strain  being  relatively  slightly  reactive 
and-the  minus  strain  unusually  reactive  during  the  early  period,  while 
the  reverse  was  true  during  the  later  part  of  the  experiment.  Such 
fluctuations  in  the  reactiveness  of  the  two  strains  of  the  same  line 
for  considerable  periods  are  seen  in  several  cases,  notably  in  Line 
695  (figure  2c)  and  Line  740  (figure  15). 

In  this  line  from  June- July  1915  to  April-May  1916  there 
was  a  general  rise  in  the  reproductive  index  for  the  plus  strain  and 
a  general  fall  in  the  reproductive  index  for  the  minus  strain.  The 
result  was  a  wide  divergence  in  reproductive  indices  for  the  two 
strains  culminating  in  April-May  1916,  when  the  reproductive  index 
for  the  plus  strain  was  nearly  twice  that  for  the  minus  strain 
(figure  12A).  This  may  be  thought  a  result  of  selection  acting  in  a 
cumulative  way  upon  the  vigor  of  the  2  strains,  but  there  was  no 
relaxation  in  or  change  in  the  method  of  selection,  and  yet  the  minus 
strain  later  reached  as  high  a  point  in  reproductive  index  as  it  had 
attained  at  any  earlier  time  (though  in  general  it  remained  lower 
than  the  plus  strain). 

While  the  reproductive  indices  indicate  (figure  12 A)  that  the 
plus  strain  was  in  general  very  much  the  more  vigorous  of  the  two  strains, 
one  fails  to  find  (as  always)  any  direct  relation  between  reproductive 
index  and  reaction-time.  The  generally  much  higher  reproductive 
index  for  the  plus  strain,  associated  as  it  is  during  most  of  the  experi- 
ment with  a  considerably  higher  reaction-time  mean  for  this  strain, 
in  itself  serves  as  a  general  denial  of  an  association  in  this  line  between 
vigor  and  reactiveness  to  light. 

LINE  796. 

This  was  a  sister  line  to  Lines  794  and  795.  Selection  was  begun 
at  the  same  time  and  continued  for  the  same  period  as  with  those 


A   PHYSIOLOGICAL  CHARACTER. 


81 


lines  (29  months),  representing  95  generations  in  the  plus  and  91 
generations  in  the  minus  strain.  The  data  are  given  in  tables  37 
and  38  and  figure  13. 

The  courses  of  the  reaction-time  curves  are  extremely  irregular, 
although  in  a  general  way  the  curves  follow  each  other  in  a  manner 
to  suggest  environmental  influence  upon  reaction-time.  It  is  obvious 
that  there  was  no  effect  of  selection. 

TABLE  37. — Selection  summary  for  Line  796. 


Time  period. 


Dec.  25,  1914- 

July  31,  1915 
Aug.  1,  1915- 

July  31,  1916 
Aug.  1,  1916- 

May  1,  1917 
Test  series, 

Aug.  1916.  .  . 
July  1,  1916- 

Sept.  30,  1916 


Strain. 


Plus.. 
Minus 
Plus.  . 
Minus 
Plus .  . 
Minus 
Plus.  . 
Minus 
Plus .  . 
Minus 


1-28 

1-27 

29-66 

28-61 

67-95 

63-90 

68 

64 

65-71 
61-68 


28 

2713 

38 

sa 

22 

20 

20 

20  32 
7  16 
714- 


?•* 

C    bD 

if 
I* 

343 

351 
604 

302 

662 
643 
116 


II 

"sr 


I! 


117 
147 
301 
175 

98 

379 
869 

76 
28 


244 
359 
364 
454 
220 
144 
256 
189 
346 


876 

857 
839 
692 
710 
900 
900 
828 


555.5 

004.5 
619.1 
697.6 
449.4 
393. 7 
675.6 

662!  4 
453.5 


289.5 
290.9 
291.9 
297.2 
331.1 
311.9 
286.6 
302.5 
201.3 
315.5 


10.5 

10.5 

8.0 

12.3 

12.8 

14-1 

7.5 

5.1 

12.6 

21.7 


s 

B 


5« 


-49.3 


+55.7 

+2i!<j 

+208 '.6 


14-9 


14-7 
19.1 


11.0 


25.1 


3.31 


5.33 
2.92 


8.31 


TABLE  38. — Same-day  broods.    Summary  of  data  for  Line  796. 


Time  period. 


Dec.  25,  1914- 
July  31,  1915 

Aug.  1,  1915- 
July  31,  1916 

Aug.  1,  1916- 
May  1,  1917 


Strain. 


Plus.... 

Minus  . . 
Plus .... 
Minus  . . 

Plus 

Minus  . . 


I 


8.4 

7. 
13.9 

5.5 
17.0 


415.5 


59 
52 
139 
55 
68 
63 


IJ 


329 
154 
382 

345 


900 
757 
900 
504 
900 
510 


1 


634 

523 
674 

723 

503 


283.10 
251.50 
176.68 
255.57 
291.84 
303.54 


24.86 
26.36 
10.11 
21.54 
23.87 
25.75 


betwe 
minus 


+111 


+220 


36.233.06 

'.24 
.26 


.070 
.146 


The  mean  reaction-times  for  the  first  longer  period  (8  months) 
of  the  experiment  were  555.5  and  604.8  seconds,  the  plus  strain 
being  the  more  reactive  by  a  margin  of  49.3  ±14.9  seconds,  a  differ- 
ence 3.31  times  the  probable  error  (table  37).  The  7  same-day 
broods  for  this  period  gave  averages  indicating  greater  reactiveness 
for  the  minus  strain  by  a  difference  of  +111  ±36.23  seconds,  more 
than  double  the  difference  obtained  from  the  entire  data  for  this 
period  and  in  the  reverse  direction  (table  38). 


82 


SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 


For  the  second  longer  period  the  reaction-time  means  were 
619.1  and  697.5  seconds.  The  difference  was  —78. 4  ±14.7  seconds, 
5.33  times  the  probable  error.  The  10  same-day  broods  gave  averages 
differing  by  only  -6  ±24.07  seconds. 

The  data  for  the  last  longer  period  (August  1916  to  the  end  of 
the  selection  experiment  in  April  1917)  gave  as  means  449.4  and 


A 

2.00 

t 

'  t 

'  I  " 

1 

*                         ( 

1.50 
1.00 

_             6                       9 

—                        o 

I 

i» 

'I- 

1 

> 

1 

• 

.50 

c 

A 

)                                                 "" 

o 

12-1          V5          8-9         12-1           <r5          8-9          IM            4 

nn 

1915                                      1916                        1917 

•it    I 


750 


600 


459 


300 


150 


B 


1916 


.75 

50 
.25 

.00 
.75 
.50 
25 
00 
.25 


FIGURE  13. — Line  796. 

A.  Reproductive  indices,  actual  values. 

B.  Reproductive  indices,  superiority. 

C.  Reaction-time  curves.     (The  asterisk  indicates  a  point 

established  from  insufficient  data,  only  one  brood.) 

393.7  seconds.  The  minus  strain  was  the  more  reactive  by  55.7  ±19.1 
seconds,  2.92  times  the  probable  error.  The  same-day-brood 
means  (only  4  broods  of  each  strain)  differed  by  220  seconds,  the 
minus  strain  being  the  more  reactive.  A  test  series  conducted  during 
August  1916  consisted  of  662  and  643  individuals  in  the  two  strains. 


A   PHYSIOLOGICAL   CHARACTER.  83 

The  means  were  675.6  and  653.7  seconds.  The  difference  was 
+21.9  ±11.0  seconds,  1.99  times  the  probable  error. 

Hence,  for  the  first  2  longer  periods  the  means  for  the  entire 
data  gave  higher  reaction-times  for  the  minus  than  for  the  plus 
strain.  These  differences  (—49.3  and  —78.4  seconds)  were  3.31  and 
5.33  times  their  probable  errors,  but  the  lack  of  confirmation  of 
these  differences  from  the  same-day-brood  data  at  once  throws  doubt 
upon  a  real  significance  for  these  differences;  while  for  the  final  long 
period  of  the  experiment  with  Line  796,  the  test  series,  the  selection 
data  as  a  whole,  and  the  same-day-brood  data  confirm  each  other  in 
indicating  a  greater  reactiveness  on  the  part  of  the  minus  strain. 

The  data  for  this  line  would  seem  to  present  these  points  of 
interest:  a  lack  of  selective  effect,  an  effect  of  environmental  in- 
fluences upon  reaction-time  as  indicated  by  the  courses  of  the  curves, 
they  following  each  other  in  a  general  way,  and  the  lack  of  any  marked 
evidence  of  a  relation  between  vigor  and  reaction-time  in  a  case  in 
which  the  two  strains  differed  extremely  in  vigor.  This  last  point 
may  be  worthy  of  a  little  further  statement. 

The  plus  strain  was  very  much  the  more  vigorous  during  most 
of  the  experiment,  yet  the  differences  in  mean  reaction-time  were 
not  such  as  to  indicate,  everything  considered,  a  significantly  higher 
reaction-time  for  the  minus  strain  than  for  the  plus  strain.  Indeed, 
the  same-day-brood  data  quite  fail  to  confirm  any  significant  dif- 
ference in  reaction-time  between  the  two  strains  for  any  part  of  the 
experiment.  During  the  3  two-month  periods  during  which  the  differ- 
ence in  mean  reproductive  indices  were  greatest  (figure  13s),  the  minus 
strain  was  less  reactive,  but  for  2  of  the  3  other  periods  (in  which  the 
inferiority  in  reproductive  index  for  the  minus  strain  was  almost  as 
great)  the  minus  strain  was  actually  the  more  reactive.  There  were 
altogether  9  periods  of  very  low  reproductive  indices  (about  1.00  or 
lower)  for  the  minus  strain,  and  these  were  periods  during  which  the 
plus  strain,  with  two  exceptions,  ran  fairly  high  in  vigor  (reproductive 
index  over  1.50),  and  hence  the  difference  in  vigor  was  marked;  yet 
the  minus  strain,  far  from  being  markedly  less  reactive,  was  during 
4  of  these  periods  actually  the  more  reactive.  It  was  the  less  re- 
active, however,  during  the  other  5  periods  and  in  3  cases  the  differ- 
ences were  large.  Though  in  general  the  reproductive  index  for  the 
minus  strain  of  this  line  remained  lower  than  that  for  the  plus  strain, 
it  regained  a  high  level  (1.7)  soon  after  this  divergence  in  reproductive 
indices  was  most  marked. 

Thus,  while  in  both  Lines  795  and  796  there  was  an  apparent 
cumulative  effect  of  selection  upon  the  vigor  of  the  minus  strain, 
in  both  cases  the  effect  was  lost  and  the  minus  strain  later  attained 
relatively  high  reproductive  indices,  although  the  method  and  pro- 
cedure of  selection  were  in  no  wise  modified. 


84 


SELECTION   IN    CLADOCERA    ON   THE    BASIS    OF 


LINE  740. 

Line  740  was  the  first  line  of  S.  exspinosus  subjected  to  selection. 
It  was  obtained  in  August  1912,  from  Pond  IV,  in  which  S.  exspinosus 
occurs  the  year  round.  Selection  was  begun  at  once  and  continued 
for  181  and  184  generations,  for  a  period  of  more  than  56  months, 

TABLE  39. — Selection  summary  for  Line  740. 


•i  . 

| 

jj 

o 

& 

| 

1 

d 

• 

t 

8 

i 

3  ii 

1  a 

1 

9  "a 

'•§ 

'•§ 

1 

a 

2 

2 

• 

1 

i 

|| 

1-3 

S 

1 

lj 

2 

i 

§ 

a 

d 

1 

i 

jS 

•a 

1 

•o  ! 

1*8 

£ 

3*0 

I 

3 

3 

I 

"o 

1 

•3 

3 

Time  period. 

Strain. 

0 

d  *o 

dl 

1. 

I-8 

15 

1 

a 

1 

.S 

1 

1 

li 

1 

L 

• 

xf 

g>  g 

IS 

o  § 

« 

s 

1 

3 

§  "? 

3 

s  2 

§ 

"o 

•  JB 

a5   hi 

1* 

o| 

6  1 

§  i 

§  i 

a  a> 

a 

1 

o 

II 

1 

ij 

* 

^ft 

I*3 

|B 

$'«• 

$* 

!* 

1 

£ 

5s 

£ 

Q3 

Aug.  27,  1912- 

(  Plus... 

1-34 

37 

11.5 

427 

0 

291 

384 

872 

735.2 

266.0 

8.7 

July  31,  1913 

(  Minus  . 

1-34 

S61 

9.9 

357 

0 

399 

757.0 

244-7 

8.7 

"-21.  8 

'12'.  3 

1.77 

Aug.    1,  1913- 

(  Plus... 

35-76 

40 

10.6 

424 

5 

184 

359 

881 

597.1 

298.7 

9.8 

July  31,  1914 

I  Minus  . 

35-79 

45 

10.7 

4S2 

2 

275 

S70 

656.9 

302.4 

9.3 

'  -69.8 

13.5 

4.43 

Test    series, 

65 

23 

28.4 

653 

1 

358 

173 

900 

666.4 

294.6 

7.8 

Apr.  1914.  .. 

\  Minus  . 

00 

23 

26.7 

614 

1 

391 

243 

900 

711.9 

278.3 

7.0 

-45.5 

10.9 

4.17 

Mar.  1914- 

(  Plus 

61-68 

8 

11.0 

88 

1 

36 

347 

870 

569.2 

May  1914. 

{  Minus 

62-71 

10 

12.6 

126 

o 

56 

269 

834 

596  S 

—27  1 

Aug.    1,  19ii^- 

(  Plus..'. 

77-118 

42 

452 

16 

244 

305 

886 

642^5 

'307  '.6 

9.8 

+34.8 

i3.8 

2.52 

July  31,  1915 

(  Minus  . 

79-119 

40 

11^4 

454 

4 

20S 

351 

S39 

607.7 

306.0 

9.7 

Aug     1,  1915- 

(    plug 

119-153 

35 

14  3 

471 

2 

227 

303 

828 

574.6 

335.6 

10.4 

July  31,  1916 

i  Minus  . 

120-155 

35 

15.3 

290 

S20 

600.0 

326.9 

9.7 

-25.4 

14-2 

1.79 

Test  series, 

|  Plus... 

142 

34 

31.7 

1078 

5 

628 

202 

900 

641.6 

324.6 

6.7 

+  63.7 

9.5 

6.71 

Mar.    1916.. 

(  Minus 

143 

34 

32.0 

10S9 

7 

523 

17S 

S72 

577.9 

331.7 

6.8 

Feb.   1916- 

(   Plug 

139-146 

8 

17.9 

143 

2 

82 

359 

900 

653.8 

'  +  12  6 

Apr.  1916.  .  . 

I  Minus  . 

140-147 

§ 

17.5 

140 

34S 

SSI 

641  .8 

Test   s  G  r  i  6  s  i 

151 

32 

38  5 

1231 

4 

199 

113 

817 

420.2 

318.8 

6.1 

June  1916... 

f  Minus  '. 

152 

Si? 

40.1 

12S2 

0 

243 

127 

S70 

525.7 

468.6 

S.S 

-105.5 

10.3 

10.24 

May    1916- 

147-153 

6 

14.3 

86 

o 

45 

234 

725 

574.7 

July  31,  1916 

I  Minus  . 

148-164 

c 

18.6 

111 

3 

75 

S39 

710.4 

—135.7 

Aug     1    1916- 

154-181 

26 

12  0 

252 

3 

27 

113 

631 

311   1 

243.0 

10.3 

May  1,  1917 

(  Minus'. 

155-184 

24 

13^5 

324 

0 

153 

669 

395.  S 

334.0 

12.5 

-S4-7 

16.2 

5.23 

Test  series 

/    pliia 

179 

26 

29  1 

756 

1 

38 

79 

716 

314  5 

256.3 

6.3 

Apr.  1917... 

1  Minus'. 

181 

26 

29.1 

756 

0 

31 

106 

67S 

329.  S 

267.0 

6.5 

-15.3 

9.1 

1.6S 

Mar.    1917- 

f  Plus... 

175-181 

6 

12.7 

76 

0 

8 

102 

560 

262.0 

248.8 

19.2 

+96.3 

21.1 

4.56 

May  1,  1917 

I  Minus  . 

178-184 

7 

13.6 

95 

0 

59 

421 

165.7 

124.6 

8.6 

TABLE  40. — Same-day  broods.    Summary  of  data  for  Line  740. 


1 

.£3 

3 

i 

jj 

i 

d 

g 

S 

i 

a1 
§ 

•i 

1 

|1 

1 

•3 

§ 
S 

s  a 

£•3 

g 
& 

Time  period. 

Strain. 

<n 

mber  of 

1 

•» 

|° 

1 

S 

i 

ividual 

| 

s 

i 

between 
ninus  sti 

i 

1 

"o 
(d 

Average  nu 
brood. 

1 

•3 
6 

No.  of  negj 
dividuals. 

•sg 

Average  EC 
time. 

Average  m 
time. 

a  o3 

Standard  c 

Probable  e 

Difference 
plus  and  i 

Probable  e 

1  Difference 
ble  error. 

Aug.    1,  1912- 

Plus 

f, 

10  0 

60 

o 

38 

386 

900 

713 

+  31 

July  31,  1913 

Minus  . 

0 

8  2 

49 

o 

29 

3S2 

900 

682 

Aug.     1,  1913- 

Plus 

c» 

10  8 

97 

1 

46 

498 

874 

607 

298  53 

20  44 

July  31,  1914 

Minus  .  . 

.9 

12.4 

112   . 

0 

54 

4/2 

7S9 

032 

293.38 

18.70 

-25 

31.18 

O.SO 

Aug.     1,  1914- 

Plus 

•j 

12  3 

37 

o 

31 

359 

900 

834 

+  19 

July  31,  1915 

"f 

10  0 

SO 

o 

21 

529 

900 

815 

Aug.     1,  1915- 

Plus    . 

14 

15  4 

216 

o 

99 

266 

782 

558 

335  22 

15  38 

July  31,  1916 

Minus  .  . 

14 

/6./ 

220 

0 

324 

806 

040 

335.71 

15.06 

-S2 

21.53 

S.SO 

Aug.     1,  1916- 

Plus.... 

4 

10.5 

42 

0 

6 

111 

818 

358 

262.16 

27.29 

+  137 

34.03 

4.02 

May    1,  1917 

Minus  .  . 

4 

10.5 

42 

0 

f 

75 

736 

221 

195.36 

20.33 

A   PHYSIOLOGICAL   CHARACTER.  85 

until  the  experiment  was  completed.  Both  strains  are  still  in  our 
laboratory.1  The  data  are  summarized  in  tables  39  and  40  and  figures 
14  and  15. 

The  first  longer  period  of  the  experiment,  approximately  11 
months,  contains  the  data  for  37  and  36  broods  of  the  two  strains. 
The  mean  reaction-times  (427  and  357  individuals)  are  735.2  and 
757  seconds  (table  39).  The  difference  (-21.8 ±12.3  seconds)  was 

1.77  times  the  probable  error.     There  were  6  same-day  broods  for 
this  period  (table  40)  for  which  the  mean  reaction-times  were  713 
and  682  seconds,  the  difference  being  +31  seconds. 

For  the  second  year-period  of  selection  with  Line  740,  the  mean 
reaction- times  (424  and  482  individuals)  were  597.1  and  656.9 
seconds.  The  difference  was  —59. 8  ±13. 5  seconds.  This  difference 
was  4.43  times  the  statistical  probable  error.  The  9  same-day 
broods  for  this  period  gave  as  mean  reaction-times  607  and  632 
seconds,  the  difference  being  —25  ±3 1.1 8  seconds. 

A  test  series  was  conducted  in  April  1914.  There  were  653 
individuals  of  the  plus  strain  and  614  of  the  minus  strain.  The  mean 
reaction-times  were  666.4  and  711.9  seconds.  The  difference  was 
—45. 5  ±10.9  seconds,  or  4.17  times  the  probable  error.  There  is 
a  suggestion  of  an  effect  of  selection  for  this  year-period  in  that  the 
data  as  a  whole,  the  data  for  the  same-day  broods,  and  the  test- 
series  data  all  show  higher  reaction-times  for  the  minus  strain.  But 
in  view  of  the  later  data  this  evidence  is  overruled. 

For  the  next  year-period  (August  1914-July  1915)  the  mean 
reaction-times  (452  and  454  individuals)  were  642.5  and  607.7.  The 
difference  (+34.8  ±13.8  seconds)  is  2.52  times  the  probable  error. 
Hence  the  difference  is  of  statistical  interest,  but  is  opposed  to  an 
effect  of  selection.  There  were  only  3  same-day  broods  for  this  year- 
period  for  which  the  mean  reaction-times  were  834  and  815  seconds. 
The  difference  was  +19  seconds. 

For  the  year-period  (August  1915- July  1916)  the  mean  reaction- 
times  (471  and  519  individuals)  were  574.6  and  600.0  seconds.  The 
difference  (-25.4  ±14.2  seconds)  is  only  1.79  times  the  probable 
error.  There  were  14  same-day  broods  for  this  period,  for  which  the 
mean  reaction-times  were  558  and  640  seconds.  The  difference  was 
-82  ±21. 53  seconds. 

A  test  series  was  conducted  during  March  1916  and  contained 

1.078  individuals  in  the  plus  strain  and  1,089  in  the  minus  strain. 
The  mean  reaction-times  were  641.6  and  577.9  seconds.    The  differ- 
ence (+63.7  ±9.5  seconds)  is  6.71  times  the  probable  error.    This 
test  series  was  conducted  with  great  care,  and  the  fact  that  the  plus 
strain  was  the  less  reactive  of  the  two  by  a  margin  of  statistical 
significance   largely  counterbalances    the    small    difference   in    the 

1  It  was  in  one  of  the  substrains  of  this  line  (740  -  Special  B)  that  the  sex-intergrades  were 
first  discovered. 


86  SELECTION   IN   CLADOCERA   ON   THE   BASIS   OF 

opposite  direction  for  the  selection  data  as  a  whole  and  the  larger 
difference  in  the  opposite  direction  for  the  same-day-brood  data  for 
this  year.  Another  test  series  was  conducted  during  this  period, 
during  June  1916;  it  contained  1,231  individuals  in  the  plus  strain 
and  1,282  in  the  minus  strain.  The  mean  reaction-times  were  420.2 
and  525.7  seconds.  The  difference  (-105.5  ±10.3  seconds)  is  10.24 
times  the  probable  error.  The  result  of  this  test  series,  conducted 
only  3  months  later  than  the  earlier  one,  is  quite  contradictory  to  the 
result  of  the  former. 

The  results  of  these  test  series  are  somewhat  puzzling,  but  table 
39,  giving  the  selection  summary,  shows  that  for  the  three-month 
period  February- April  1916  (including  the  month  in  which  the 
earlier  test  series  was  made  and  that  just  preceding  and  just  succeed- 
ing) the  mean  reaction-time  was  much  higher  for  the  plus  strain.  The 
similar  period  of  3  months,  May-July  1916,  during  which  the  June 
test  series  was  conducted,  shows  a  mean  reaction-time  in  the  minus 
strain  135.7  seconds  greater  than  that  for  the  plus  strain.  One  sees, 
in  these  test  series  and  the  selection  data  for  the  contingent  periods, 
that  the  relative  reactiveness  had  changed  here  in  a  comparatively 
short  time.  Reference  to  the  curve,  figure  15,  shows  that  for  the 
two-month  period,  February-March  1916,  the  two  curves  met,  but 
for  the  later  two-month  periods  the  minus  strain  for  a  considerable 
time  was  much  less  reactive  than  the  plus  strain. 

For  the  final  longer  period  of  the  experiment  with  Line  740 
(August  1916  to  the  close  of  the  experiment,  May  1,  1917)  the  mean 
reaction-times  were  311.1  seconds  and  395.8  seconds  for  the  two 
strains.  The  difference  (—84.7 ±16. 2  seconds)  was  5.23  times  the 
probable  error.  There  were  only  4  same-day  broods,  for  which  the 
mean  reaction- times  were  358  and  221  seconds,  the  difference  being 
+ 137  ±34. 03  seconds.  Here  the  number  of  same-day  broods  was 
too  small  to  be  given  serious  consideration,  but  it  is  interesting  to 
note  that  with  these  same-day  broods  the  plus  strain  was  so  much 
less  reactive  than  the  minus,  in  opposition  to  the  difference  shown 
by  the  data  as  a  whole  for  this  period.  Two  of  these  4  same-day 
broods,  however,  occurred  during  the  final  3  months  of  the  experi- 
ment, when  the  data  as  a  whole  showed  that  the  minus  strain  was 
temporarily  the  more  reactive. 

Reference  to  the  curves  of  the  mean  reaction- times  (figure  15) 
shows  that  for  10  months  (from  April  1916  to  January  1917,  inclusive) 
the  minus  strain  was  the  less  reactive  and  that  the  margin  of  differ- 
ence increased  until  in  the  December  1916  and  January  1917  periods 
the  reaction- time  for  the  minus  strain  was  more  than  double  that 
for  the  plus  strain.  The  data  for  a  portion  of  this  ten-month  period 
was  not  very  satisfactory.  We  were  troubled  with  poor  food  con- 
ditions, and,  at  times,  when  the  stock  was  not  well  nourished,  the 
selections  were  not  made  by  tests,  but  individuals  were  chosen  at 


A   PHYSIOLOGICAL   CHARACTER. 


87 


random.  Because  of  this  fact,  the  data  for  4  months  (October  1916- 
January  1917)  are  not  very  complete  and  the  differences  for  those 
2  two-month  periods  are  not  to  be  credited  to  the  same  extent  as 
the  differences  for  the  remainder  of  the  experiment. 

Reference  to  tables  48  and  49  (columns  5  and  6)  and  figure  15 
and  18s  shows  that  this  consistent  divergence  in  reactiveness  be- 


oo 


50 


00 


50 


00 


8- 


-! 


8-9        12- 
1912 


4-5        8-9        12-1        4-5       8-9 
1913  1914 


12-1 


4-5       8-9        12-1        4-5       8-9 
1915  1916 


12-1 


4 
1917 


75 

50 
25 
00 

25 
.50 
.75 


300 


1*1 


FIGURE   14. — Line  740. 
A.  Reproductive  indices,  actual  values.        B.  Reproductive  indices,  superiority. 


8-9         12-1        4-5        8-9        12-1        4-5       8-9        12-1        4-5        8-9        12-1        4-5        8-9        12-1 
1912  1913  1914  1915  1916 


FIGURE  15. — Line  740. 
Reaction-time  curves  with  reaction-time  curves  for  Line  757  superimposed. 


88  SELECTION   IN   CLADOCERA   ON   THE   BASIS   OF 

tween  the  two  strains  of  Line  740  for  the  period  April  1916-January 
1917  is  due  to  exceptionally  high  means  for  the  minus  strain  during 
the  period  April-September  and  to  exceptionally  low  means  in  the 
plus  strain  from  October  to  the  following  January,  both  relative  to 
the  means  for  corresponding  two-month  periods  for  the  other  S. 
exspinosus  strains  (Lines  794,  795,  and  796).  The  following  of  one 
of  these  two  exceptional  reaction- time  periods  just  upon  the  other 
produces  the  long  period  of  divergence  which  at  first  sight  seems  very 
suggestive  of  an  effect  of  selection.  Since  it  is  seen  in  this  case  that 
neither  the  plus  nor  the  minus  strain  of  Line  740  is  consistently 
different  from  the  composite  curve  for  the  other  plus  and  minus 
strain  of  lines  of  S.  exspinosus  throughout  this  ten-month  period,  it 
seems  pretty  evident  that  genetic  change  is  not  responsible  for  this 
divergence.  It  is  believed  that  the  explanation  lies  in  the  unavoid- 
able differential  treatment  of  the  two  strains  of  the  same  line  (see 
pages  140-142). 

A  final  test  series  was  conducted  in  April  1917.  There  were  756 
individuals  of  each  strain.  The  mean  reaction-times  were  314.5 
seconds  and  329.8  seconds.  The  difference  (  —  15. 3  ±9.1  seconds) 
was  only  1.68  times  the  probable  error.  For  the  contingent  two- 
month  period  the  plus  strain  was  the  less  reactive  by  96.3  seconds. 
This  test  series,  fortunately  conducted  at  the  close  of  selection, 
showed  conclusively  that  there  was  no  effect  of  selection  in  this 
line;  or,  if  there  had  actually  been  an  effect  of  selection,  that  it 
had  been  lost.  The  selections  were  somewhat  relaxed  during  the 
October-January  period,  but  it  does  not  seem  probable  that  a  real 
selective  difference  would  have  been  lost  so  quickly.  Hence,  every- 
thing considered,  the  writer  does  not  believe  that  the  data  for  the 
ten-month  period  in  which  there  was  a  consistent  reaction-time 
difference  between  the  two  strains  indicates  an  effect  of  selection. 

Line  740  is  somewhat  unique  among  the  lines  of  Cladocera  for 
which  the  data  for  vigor  has  been  worked  out,  in  that  the  minus 
strain  was  on  the  whole  more  vigorous.  Of  the  29  two-month  periods, 
the  minus  strain  was  the  more  vigorous  as  measured  by  the  repro- 
ductive index  for  18  periods  and  the  plus  strain  was  the  more  vigorous 
of  the  two  for  only  11  periods.  The  differences  in  favor  of  the  plus 
strain  average  somewhat  larger  than  those  favoring  the  minus  strain, 
however.  Examining  these  data  with  reference  to  periods  of  superior- 
ity in  the  reproductive  index  on  the  part  of  the  two  strains,  one  fails 
to  find  any  correlation  between  relatively  greater  reactiveness  and 
greater  vigor.  For  the  5  two-month  periods  during  which  the  re- 
productive index  for  the  plus  strain  was  superior  to  that  for  the 
minus  strain  by  0.25  or  more,  the  plus  strain  had  a  higher  reaction- 
time  twice  and  a  lower  reaction-time  three  times.  For  the  6  two- 
month  periods  for  which  the  reproductive  index  for  the  minus  strain 
was  superior  to  that  for  the  plus  strain  by  0.25  or  more,  the  reaction- 


A   PHYSIOLOGICAL   CHARACTER.  89 

time  mean  for  the  minus  strain  was  higher  four  times  and  lower 
twice.  Further  detailed  analysis  fails  to  show  any  relation  between 
mean  reproductive  index  and  mean  reaction-time. 

The  curves  for  Line  740  indicate  clearly  a  marked  general  effect 
of  environmental  conditions  upon  the  reactiveness  of  these  animals. 
There  are  irregularities  in  the  curve,  but  in  a  general  way  the  reaction- 
time  curves  for  the  two  strains  follow  each  other  remarkably  well. 

It  is  possible  that  a  mutation  occurred  in  the  minus  strain  of 
Line  740  in  the  forty-third  generation  of  selection.  A  brood  of  8 
young  on  September  28,  1913,  had  the  remarkably  low  reaction- 
time  mean  of  159  seconds.  These  were  day-old  young,  however,  and 
day-old  young  are  sometimes  abnormally  reactive.  Further,  4  of 
these  8  young  were  negative  in  their  reactions,  an  unusual  result, 
indicating  unusual  experimental  conditions  (see  page  15).  However, 
on  the  following  day  a  brood  of  8  from  another  mother  of  this  strain 
had  a  mean  of  only  280  seconds. 

Examination  of  the  other  selection  data  for  the  same  days 
showed  that  the  broods  of  the  strains  of  D.  pulex  from  which  selections 
were  made  were  unusually  reactive.  Fairly  extensive  tests  of  addi- 
tional broods  of  740  minus  and  other  strains  of  S.  exspinosus  extend- 
ing over  several  generations  failed  to  show  any  consistently  greater 
reactiveness  in  740  minus,  though  certain  descendants  of  the  very 
reactive  individuals  of  the  minus  strain  of  Line  740  produced  broods 
some,  but  a  minority,  of  which  were  very  reactive. 

The  results  of  these  tests  and  the  fact  of  the  unusual  reactive- 
ness  of  many  of  the  broods  of  other  strains  tested  during  this  period 
led  to  the  conclusion  that  a  mutation  had  probably  not  occurred  and 
that  the  unusual  reactiveness  of  the  minus  strain  of  Line  740  at  that 
time  had  probably  been  caused  by  unusual  experimental  conditions. 

If  a  mutation  did  occur  it  should  have  remained  apparent  (in 
the  absence  of  segregation)  at  least  for  a  considerable  period,  in 
spite  of  any  effect  of  selection.  But  after  the  second  generation  the 
reaction-means  for  740  minus  were  normal,  running  usually  above 
700  seconds. 

LINE  757. 

PRESENTATION  OP  DATA. 

The  data  for  Line  757  will  be  examined  with  great  care,  for  with 
this  line  the  result  is  different  from  that  with  most  of  the  other  lines 
used  in  these  selection  experiments,  and  it  is  desired  to  be  conserva- 
tive regarding  any  conclusions  that  may  be  reached  regarding  the 
effect  of  selection  within  this  line. 

The  original  mother  of  Line  757  was  obtained  October  19,  1912, 
from  Pond  I,  the  temporary  surface-water  pond.  Selection  was 
begun  with  her  first  laboratory  brood  of  young  and  continued  for  181 
generations,  covering  a  period  of  4^  years. 


90  SELECTION    IN    CLADOCERA    ON   THE    BASIS    OF 

The  tabulations  of  the  data  and  the  diagrams  employed  are 
practically  identical  with  those  used  in  presenting  the  data  for  Line 
695  and  the  other  lines  to  which  rather  careful  treatment  has  been 
accorded.  They  are:  tabulations  of  the  data  by  broods  for  the  757 
plus  strain  and  the  757  minus  strain  (tables  41  and  42) ;  tabulation 
by  two-month  periods  (tables  43  and  44) ;  a  summary  of  the  selection 
data,  including  the  results  of  the  test  series  (table  45);  a  tabulation 
of  the  data  for  the  same-day  broods  (table  46) ;  summary  of  data  for 
more  reactive  individuals  (table  47) ;  figure  18s,  showing  graphically 
the  mean  reaction-times  of  the  two  strains  by  two-month  periods; 
figure  19,  showing  the  reaction-time  curves  by  six-month  periods; 
figure  16,  showing  the  relative  rates  of  descent  of  the  two  strains; 
figure  17 A,  representing  the  average  brood-size  for  the  two  strains; 
figure  17s,  showing  average  age  of  mothers  at  the  time  the  first 
broods  were  produced;  figure  17c,  giving  the  reproductive  indices 
for  the  two  strains;  and  figure  ISA,  showing  graphically  the  differ- 
ences in  vigor  of  the  two  strains  as  indicated  by  the  reproductive 
indices.  .Tables  48  and  49,  comparing  the  data  for  Line  757  with  that 
for  the  other  S.  exspinosus  lines,  are  also  presented,  and  the  data  in 
them  are  graphically  presented  in  figures  15  and  18B. 

Examination  of  tables  41  and  42  shows  that  in  Line  757,  as  with 
all  the  lines  subjected  to  selection,  there  are  considerable  fluctuations 
in  the  reactiveness  of  individual  broods.  During  the  first  5  months 
of  selection  the  range  in  mean  reaction-time  by  broods  for  the  plus 
strain  was  from  472  to  900  seconds  and  for  the  minus  strain  from 
586  to  900  seconds.  The  fluctuations  in  reaction-time  means  by 
broods  were  still  large  after  the  two  strains  became  widely  different 
in  their  reactiveness  to  light.  For  the  last  5  months  of  selection  with 
Line  757  the  plus  strain  means  ranged  from  93  to  651  seconds  and 
the  minus  strain  means  from  133  to  814  seconds. 

Although  brood-by-brood  comparison  (tables  41  and  42)  shows 
clearly  that  in  general,  after  the  early  part  of  the  selection  experi- 
ment with  Line  757,  the  minus  broods  were  very  much  the  less  re- 
active, the  rather  wide  fluctuations  by  brood  averages  made  it  desir- 
able to  treat  the  data  by  larger  units. 

Tables  of  the  data  by  one-month  periods  were  made.1  From 
the  averages  in  these  tables  it  was  found  that  for  the  first  9  months 
of  selection  the  plus  strain  of  Line  757  was  the  more  reactive  for  7 
months;  for  the  second  longer  period  (the  year  August  1913-July 
1914)  the  plus  was  the  more  reactive  during  10  months;  for  the  third 
longer  period  the  plus  was  the  more  reactive  during  10  months;  for 
the  remaining  21  months  of  the  experiment  the  plus  was  the  more 
reactive  during  each  month.  After  the  first  5  months  of  selection 

1  These  are  omitted  from  the  paper  for  the  sake  of  reducing  the  cost  of  publication  and  in 
favor  of  retaining  in  the  paper  the  tables  of  data  by  broods  in  which  the  original  data  are  given 
in  sufficient  detail  to  make  possible  their  utilization  by  anyone  who  might  wish  to  examine  the 
data  statistically. 


A   PHYSIOLOGICAL   CHARACTER. 


91 


TABLE  41. — Summary  of  selection  data  by  broods  for  Line  757  plus. 


a 

A 

3 

1 

0 

B 

B 

.c 

4 

a 

B 

B 

o 

? 

B 

• 

9 

S 

s  _« 

S 

o> 

a 

01 

i 

0 

I 

"o 

'S 

a 

iS  (D 

'•3 

'"V 

.§ 

B 

E 

£ 

i 

"3 

**  CO 

«•! 

.0 

.s 

M 

S 

^5 
S-8 

a 
.2 

G 
0 

1 

| 

1 

•o 

£J 

I 

1 

|.a 

B 

1 

ta 

l-s 

*r3  ^ 

2 

g 

I 

| 

1 

i 

|j 

"8 

•4 

O  ^3 

>>  a 
"8.1 

g 

ft 

"o 

L 

J| 

•9g 

°-g 

g  ^ 
Bg 

|| 

°l 

1 

g1 

°S 

• 

rt  -fl 

§ 

III 

6 

g  B 
* 

d'~ 

6  w 

is  s 

S  § 

a  | 

g 

s.§ 

O 

Q 

W 

<  ft 

S'0 

§  °° 

s  " 

GO  " 

S 

i* 

1912 

A 

Nov.  16 

2 

0 

3 

15.0°C. 

0 

3 

900 

900 

2700 

900 

2430000 

B 

Nov.  26 

2 

0 

3 

16.4 

0 

1 

90 

900 

1415 

472 

998725 

C 

Dec.  11 

1 

0 

9 

19 

0 

8 

250 

900 

7450 

828 

6542500 

D 

Dec.  21 

11 

0 

7 

16 

0 

4 

105 

900 

4030 

576 

3308350 

E 

Dec.  30 

2 

0  or  1 

11 

16 

0 

6 

130 

900 

7525 

684 

5916475 

1913 

F 

Jan.  7 

2 

0 

28 

17 

0 

20 

70 

900 

20790 

743 

17572800 

G 

Jan.  15 

2 

0 

9 

15 

0 

4 

130 

900 

6025 

669 

4805025 

H 

Jan.  22 

2 

0 

12 

14.1 

0 

12 

900 

900 

10800 

900 

9720000 

I 

Jan.  30 

1 

0 

6 

15.4 

0 

5 

130 

900 

4630 

772 

4066900 

J 

Feb.  10 

2 

0 

11 

10.4 

0 

9 

260 

900 

8860 

886 

76076000 

K 

Feb.  19 

2 

0 

12 

12.8 

0 

9 

250 

900 

9110 

759 

7642100 

L 

Feb.  28 

2 

0 

15 

16.2 

0 

9 

250 

900 

11325 

755 

9276025 

M 

Mar.  10 

1 

0  or  1 

15 

15 

0 

12 

195 

900 

12070 

805 

10354850 

N 

May  19 

3 

0 

8 

13.1 

0 

5 

585 

900 

6540 

818 

5459850 

0 

Mar.  28 

4 

0 

11 

13.3 

0 

11 

900 

900 

9900 

900 

8910000 

P 

April  7 

5 

Oor  1 

10 

14.8 

0 

7 

205 

900 

7295 

730 

6021675 

§ 

April  14 
April  22 

4 
4 

0  or  1 
0 

13 
14 

18 
18 

0 
0 

11 

14 

300 
900 

900 
900 

10530 
12600 

810 
900 

9108900 
11340000 

S 

April  29 

3 

0 

12 

16 

0 

6 

420 

900 

8330 

694 

6311900 

T 

May  7 

2 

0 

14 

17 

0 

14 

900 

900 

12600 

900 

11340000 

U 

May  17 

3 

0 

19 

16.2 

0 

14 

252 

900 

14277 

761 

11975079 

V 

June  4 

5 

0 

11 

19.2 

0 

1 

275 

900 

5185 

471 

3018275 

w 

June  13 

2 

0 

12 

17.5 

0 

12 

900 

900 

10800 

900 

9720000 

X 

June  20 

2 

0 

14 

19.5 

0 

13 

580 

900 

12285 

877 

10872225 

Y 

June  26 

3 

0 

12 

20.6 

0 

12 

900 

900 

10800 

900 

9720000 

Z 

July  3 

3 

0 

8 

22.3 

0 

7 

720 

900 

7020 

878 

6188400 

As 

July  9 

2 

0 

12 

21 

0 

12 

900 

900 

10800 

900 

9720000 

Ba 

July  18 

4 

0 

3 

22 

0 

3 

900 

900 

2700 

900 

2430000 

July  30 

4 

0 

6 

23 

1 

3 

260 

900 

3850 

642 

2935700 

Di 

Aug.  7 

3 

0 

11 

21.5 

0 

11 

900 

900 

9900 

900 

8910000 

E, 

Aug.  13 

5 

0 

10 

20.7 

0 

9 

360 

900 

8460 

846 

7419600 

F» 

Aug.  19 

7 

0 

17 

23. 

0 

17 

900 

900 

15300 

900 

13770000 

Gs 

Aug.  26 

4 

0 

8 

20 

0 

1 

235 

900 

4000 

600 

2400850 

Hs 

Sept.  1 

2 

0 

16 

21.7 

0 

1 

92 

900 

4003 

250 

1519785 

Is 

Sept.  8 

11 

2 

11 

22 

1 

5 

105 

900 

5970 

643 

4613358 

Js 

Sept.  16 

10 

0 

14 

13.9 

0 

11 

251 

900 

11054 

790 

9449526 

K, 

Sept.  22 

5 

Oor  1 

20 

21 

0 

15 

467 

900 

16342 

817 

13824500 

L» 

Sept.  29 

10 

2 

13 

16.5 

0 

5 

192 

900 

6957 

6S5 

4981437 

Mi 

Oct.   6 

11 

0 

14 

17.4 

0 

11 

343 

900 

11052 

789 

9355370 

Oct.  13 

11 

1 

8 

16.6 

0 

0 

63 

670 

2980 

373 

1414066 

0s* 

Oct.  20 

3 

0 

8 

18.8 

0 

6 

385 

900 

6500 

813 

5519450 

Ps 

Oct.  31 

2 

0 

7 

14.5 

0 

6 

600 

900 

6000 

857 

5220000 

Q* 

Nov.  10 

11 

1 

5 

16 

0 

1 

161 

900 

2516 

503 

1703646 

Rs 

Nov.  21 

11 

0 

8 

13.7 

0 

6 

302 

900 

6061 

758 

5080085 

Si 

Dec.  1 

11 

1 

14 

16.5 

0 

0 

118 

548 

2905 

207 

776709 

Ts 

Dec.  8 

11 

0 

9 

13.4 

0 

4 

191 

900 

5978 

664 

4609610 

Us 

Dec.  19 

3 

0 

6 

12 

0 

2 

480 

900 

4394 

732 

3354148 

V, 

Dec.  30 

1 

0 

4 

10.5 

0 

2 

160 

900 

2240 

560 

1724000 

1914 

Wi 

Jan.   7 

2 

0 

8 

12.4 

0 

0 

192 

538 

2663 

333 

963695 

Xs 

Jan.  17 

4 

0 

11 

15.3 

0 

8 

810 

900 

9700 

882 

8564950 

Ys 

Jan.  24 

2 

0 

18 

15.8 

0 

12 

115 

900 

12931 

718 

10619473 

ft 

Jan.  30 

1 

0 

12 

15.9 

0 

5 

163 

900 

7234 

603 

5394950 

Ai 

Feb.  6 

5 

0 

13 

16.2 

0 

13 

900 

900 

11700 

900 

10530000 

Ba 

Feb.  14 

1 

0 

14 

17.1 

1 

11 

450 

900 

11480 

820 

9773000 

Ci 

Feb.  21 

4 

0 

20 

13.6 

0 

6 

165 

900 

9609 

481 

6265183 

Da 

Mar.  2 

5 

1 

12 

15.8 

1 

1 

116 

900 

4020 

SS5 

1990890 

Mar.  11 

1 

0 

14 

20.8 

0 

13 

390 

900 

12090 

864 

10682100 

F! 

Mar.  21 

3 

0 

12 

16.3 

0 

2 

192 

900 

5427 

452 

3241407 

G, 

Mar.  30 

11 

1 

19 

16.8 

0 

11 

223 

900 

12903 

679 

10099829 

Ha 

April  8 

2 

0 

14 

18.6 

0 

3 

95 

900 

5650 

404 

3349050 

la 

April  16 

4 

0 

12 

19.1 

0 

10 

220 

900 

9910 

826 

8624500 

Jl 

May  1 

4 

0 

11 

18.5 

0 

7 

355 

900 

8438 

767 

6880534 

K, 

May  8 

4 

0 

19 

19 

0 

10 

70 

900 

13430 

707 

10596900 

La 

May  18 

2 

1 

10 

19.2 

0 

10 

900 

900 

9000 

900 

8100000 

Ma 

May  25 

3 

0 

6 

20 

0 

6 

900 

900 

5400 

900 

4860000 

Na 

June  1 

2 

0 

11 

20.5 

0 

4 

150 

900 

6211 

565 

4363339 

0. 

June  10 

1 

0 

9 

19.8 

0 

6 

307 

900 

6544 

727 

5315338 

92  SELECTION   IN   CLADOCERA   ON   THE   BASIS   OF 

TABLE  41. — Summary  of  selection  data  by  broods  for  Line  757  plus — Continued. 


a 

A 

1 

a 

3  . 

2 

s 

.2 

1 

a 

i 

0> 

o 

M 

•3 

j 

a 
43 

c  e 
•3  3 

Q> 

_B 

a 

1 

i 

1 

2 

1 

1> 

<3 

§ 

<£  <o 

Y 

f 

•9 

.2 

B 

O 

IH 

o 

1 

a 

a 

1 

i? 

1 

| 

1 

2 
V 

"3 

1 

'S 

s! 

i 

• 

5 

?"S 

2 

2 

i 

§ 

| 

1 

o 

II 

1 

h 

Pi 

1 

!| 

is 

•-  c8 

2  oj 

3  *3 

1  « 

"o  •§ 

a 

£ 

2 

"o 

•s.a 

*0 

ftjrf 

*0  •** 

a  § 

a"c 

a 

0  8 

1 

5  w 

£j 

«  "C 

fl 

.  °> 

*  C3 

G  o 

M  g 

g  S 

S 

a  a 

I 
O 

<S5 

w 

4?S 

6 

»« 

IS 

38 

«5  M 

A 

58 

o 

I* 

1914 

p, 

June  18 

5 

0 

3 

19.0°C. 

0 

2 

305 

900 

2105 

702 

1713025 

Qi 

June  26 

11 

0 

5 

22 

0 

3 

365 

900 

3883 

777 

3232349 

S 

July  4 

11 

0 

11 

19.7 

0 

11 

900 

900 

9900 

900 

8910000 

Si 

July  13 

4 

1 

24 

21.1 

0 

24 

900 

900 

21600 

900 

19440000 

Ti 

July  18 

12 

0 

8 

24 

0 

7 

110 

900 

6410 

801 

5682100 

Ui 

July  25 

1 

0 

5 

22 

0 

4 

120 

900 

3720 

744 

3254400 

Vi 

Aug.  4 

2 

0 

12 

21.8 

1 

5 

200 

900 

6545 

545 

4688525 

Aug.  10 

3 

0 

11 

23.2 

1 

1 

140 

900 

3753 

341 

1790699 

Xi* 

Aug.  17 

2 

1 

10 

21.4 

0 

0 

162 

775 

4257 

426 

2229417 

Y, 

Aug.  22 

5 

0 

10 

22.9 

0 

9 

402 

900 

8502 

850 

7451604 

Zi 

Aug.  28 

4 

0 

10 

19.7 

0 

6 

480 

900 

7950 

795 

6533700 

Ai 

Sept.  7 

4 

1 

•40 

21.6 

0 

19 

128 

900 

24689 

617 

18907437 

B4 

Sept.  17 

11 

0 

6 

16.3 

0 

0 

170 

261 

1291 

215 

283171 

cl 

Sept.  24 

3 

0 

21 

20 

0 

8 

136 

900 

12592 

600 

9322494 

D4 

Oct.  5 

3 

1 

16 

19.6 

0 

0 

70 

570 

3352 

210 

914534 

E4 

Oct.  12 

4 

0 

12 

19.1 

1 

2 

145 

900 

5146 

429 

2869308 

F4 

Oct.  20 

3 

0 

16 

21 

0 

11 

210 

900 

11402 

713 

9395010 

G4 

Oct.  28 

5 

0 

16 

18 

1 

13 

305 

900 

12890 

806 

11021250 

1U 

Nov.  6 

3 

0 

18 

18.1 

0 

11 

90 

900 

12694 

705 

10183674 

It 

Nov.  13 

2 

0 

15 

18 

0 

13 

310 

900 

12350 

823 

10741700 

J4 

Nov.  20 

4 

0 

14 

21.1 

0 

12 

543 

900 

12131 

867 

10635793 

K4 

Nov.  28 

11 

0 

12 

18 

0 

7 

270 

900 

8275 

690 

6602575 

Dec.  4 

3 

0 

10 

18.4 

0 

10 

900 

900 

9000 

900 

8100000 

M4 

Dec.  11 

5 

0 

7 

18 

0 

7 

900 

900 

6300 

900 

5670000 

N4 

Dec.  22 

4 

0 

12 

15.7 

1 

2 

168 

900 

5319 

443 

3079429 

Dec.  31 

10 

1 

12 

15.2 

0 

6 

133 

900 

7831 

653 

6116351 

1915 

P4 

Jan.  8 

3 

0 

8 

19.6 

0 

4 

230 

900 

5656 

707 

4498818 

Q4 

Jan.  15 

2 

0 

8 

19.2 

0 

8 

900 

900 

7200 

900 

6480000 

s 

Jan.  25 

3 

0 

12 

17 

0 

7 

220 

900 

7982 

665 

6282834 

84 

Feb.  4 

4 

0 

14 

13.5 

0 

9 

65 

900 

9133 

652 

7584279 

T4 

Feb.  16 

1 

0 

12 

17.3 

0 

2 

185 

900 

5308 

442 

3055384 

U« 

Feb.  24 

4 

0 

10 

20 

0 

0 

75 

465 

1910 

191 

505550 

V4 

Mar.  6 

5 

0 

7 

19.2 

0 

0 

80 

388 

1022 

146 

315738 

W4 

Mar.  15 

3 

0 

13 

17.9 

0 

1 

136 

900 

4182 

322 

1879940 

X4 

Mar.  24 

2 

0 

10 

17 

0 

4 

185 

900 

5627 

563 

4022017 

Y4 

April  2 

10 

0 

15 

16.6 

0 

6 

120 

900 

9619 

641 

7424255 

April  12 

5 

0 

14 

20.5 

0 

13 

500 

900 

12200 

459 

10780000 

Al 

Bi 

April  21 
May  5 

8 
8 

Random 
18.9 

distr 
1 

buti( 

1 

m. 
183 

900 

4146 

518 

2654998 

2 

0 

Ci 

May  15 

3 

0 

10 

16 

0 

4 

340 

900 

6782 

678 

5107074 

Di 

May  28 

3 

0 

6 

16 

0 

0 

160 

715 

1816 

303 

767146 

Ei 

June  8 

4 

0 

16 

18.9 

0 

6 

210 

900 

8546 

534 

5928436 

Fi 

June  14 

1 

0 

17 

20.5 

0 

3 

183 

900 

7695 

453 

4353433 

Gi 

June  22 

9 

0 

9 

20 

0 

5 

400 

900 

6731 

748 

5390501 

H* 

July  1 

4 

0 

10 

20.4 

0 

3 

372 

900 

6375 

638 

4499029 

I* 

July  8 

4 

0 

11 

21 

0 

11 

900 

900 

9900 

900 

8910000 

Ji 

July  15 

3 

0 

11 

21.9 

0 

6 

120 

900 

7571 

688 

6273601 

Ki 

July  22 

4 

0 

7 

21.1 

0 

0 

115 

727 

3522 

503 

2063154 

L. 

July  30 

3 

0 

4 

23 

0 

0 

130 

390 

976 

244 

275146 

Mi 

Aug.  13 

4 

0 

6 

23 

0 

5 

410 

900 

4910 

818 

4218100 

Ni 

Aug.  19 

5 

0 

5 

19.3 

0 

3 

65 

900 

3225 

645 

2645825 

Oi 

Aug.  26 

4 

0 

7 

20.2 

0 

5 

210 

900 

4990 

713 

4172500 

P. 

Sept.  6 

6 

0 

8 

20.3 

0 

2 

200 

900 

4195 

524 

2842925 

Q* 

Sept.  12 

5 

0 

12 

21.2 

6 

3 

120 

900 

7065 

589 

5263925 

E 

Sept.  17 

4 

0 

10 

23.9 

1 

6 

180 

900 

7180 

718 

5814800 

5 

Sept.  24 

4 

0 

12 

17.3 

0 

2 

120 

900 

3670 

306 

2005500 

TI 

Oct.  4 

4 

1 

15 

17.6 

0 

7 

150 

900 

8795 

686 

6556825 

Ui 

Oct.  13 

4 

0 

13 

17.5 

0 

9 

185 

900 

9810 

755 

8112150 

V, 

Oct.  21 

5 

0 

20 

18.9 

0 

12 

70 

900 

12760 

638 

10517450 

w, 

Nov.  1 

5 

0 

22 

17.1 

0 

1 

130 

900 

5995 

£73 

2162325 

X, 

Nov.  10 

4 

0 

18 

14.1 

0 

0 

100 

390 

4455 

248 

1264675 

Y, 

Nov.  19 

2 

0 

23 

16.3 

0 

0 

145 

450 

7550 

328 

2092500 

•  Probably  two  broods,  the  first  brood  having  been  overlooked  until  the  second  appeared.  Inasmuch 
aa  the  second  brood  (if  there  were  two)  was  a  day  old  before  this  test  was  made,  the  first  brood  must  then  have 
been  near  72  hours  old.  Obviously  data  for  three-day-old  individuals  is  not  strictly  comparable  to  the  other 
data,  nearly  all  of  which  was  obtained  from  individuals  less  than  24  hours  old.  Hence,  much  as  the  writer 
dislikes  to  cull  his  data  in  any  manner,  the  data  for  this  day's  test  is  omitted.  However,  it  may  be  added  that 
this  is  the  only  instance  in  the  data  of  this  publication  in  which  any  data  has  been  rejected. 


A   PHYSIOLOGICAL   CHARACTER. 


93 


TABLE  41. — Summary  of  selection  data  by  broods  for  Line  757  plus — Continued. 


.9 
13 

2 

1 

.9 

o 

a 

a 

d 

1 

A 

s 

d 

i 
'« 

M 

.9 

3 

§ 

11 

1 

•£ 

S 

1 

a 

1 

1 

1 

2  £ 

1 

a 

i 

JS* 

a 
o 

I 

1 

1 

•3 

1 

S 

03  -T1 

.2 

a 

~ 

3  "3 

"S 

"S 

•43 

3 

8 

g 

I  . 

I 

!«• 

H 

o 

1 

ii 

jl 

N 

g   . 

a  ^ 

i 

a 

E 

S3 
1 

'43 

1 

.2  W 

•3 

•3.1 

•8 

H 

•gl 

°1 

11 

a! 

i 

d 

°  s 

a  a 

O 

Q* 

i 

S& 

£ 

gj 

t* 

ge 

|i 

|I 

CO  °° 

i 

3  '.£» 
OQ 

1915 

Zs 

Nov.  29 

4 

0 

21 

15.5°C. 

5 

2 

145 

900 

7895 

876 

3806975 

A< 

Dec.  8 

4 

0 

20 

15.8 

3 

0 

165 

615 

7115 

356 

2827225 

B8 

Dec.  17 

4 

0 

23 

20.2 

1 

0 

120 

580 

6145 

267 

1866475 

C« 

Dec.  27 

5 

1 

21 

14.9 

5 

1 

95 

900 

5400 

257 

2031375 

1916 

D8 

Jan.   7 

8 

0 

5 

16.5 

0 

5 

900 

900 

4500 

900 

4050000 

E« 

Jan.  17 

9 

0 

6 

16.8 

0 

0 

140 

240 

1105 

184 

210725 

F. 

Jan.  26 

5 

0 

20 

15.1 

0 

1 

110 

900 

5420 

271 

2113350 

G» 

Feb.  4 

9 

0 

17 

12 

0 

4 

60 

900 

7060 

415 

4527250 

H, 

Feb.  15 

8 

0 

15 

16.6 

0 

6 

200 

900 

7750 

517 

5206900 

It 

Mar.  !• 

9 

0 

26 

17 

0 

26 

900 

900 

23400 

900 

21060000 

Je 

Mar.  13 

2 

1 

17 

18 

0 

3 

120 

900 

6225 

366 

3436175 

K. 

Mar.  22 

4 

0 

19 

15.9 

0 

3 

90 

900 

5690 

379 

3423300 

L« 

Mar.  31 

9 

0 

5 

15.1 

1 

0 

135 

580 

1770 

854 

756750 

Mab 

May  4 

7 

0 

4 

14 

0 

3 

190 

900 

2890 

723 

2466100 

N» 

May  15 

4 

0 

24 

15.8 

0 

3 

170 

900 

8625 

359 

2625625 

o« 

May  25 

3 

0 

17 

18.5 

0 

16 

210 

900 

14610 

859 

13004100 

P. 

June  8 

3 

7 

19.6 

0 

4 

175 

900 

4370 

624 

3455450 

Q< 

June  19 

4 

15 

20.2 

0 

5 

270 

900 

8805 

587 

6066825 

Rd 

June  28 

5 

13 

20.6 

0 

0 

75 

805 

4483 

345 

2002129 

July  5 

4 

11 

18.9 

0 

1 

110 

900 

3560 

324 

1777550 

Td 

July  12 

5 

11 

23.7 

0 

1 

150 

900 

3520 

320 

1565800 

U. 

July  18 

3 

1 

9 

21.5 

0 

0 

130 

330 

2060 

229 

506400 

V8 

July  24 

4 

1 

8  ' 

22 

0 

4 

90 

900 

4390 

549 

3418900 

w« 

Aug.  1 

3 

0 

19 

20.6 

0 

6 

85 

900 

10240 

539 

6987650 

X. 

Aug.  9 

Selection  made 

>ut  record  incom 

te. 

Ye 

Aug.  16 

11 

0 

14 

19.9 

0 

1 

120 

900 

4110 

294 

1737800 

Z, 

Aug.  22 

3 

0 

8 

21.8 

0 

8 

90 

900 

7200 

900 

6480000 

AT 

Aug.  28 

3 

1 

14 

18.5 

0 

1 

90 

900 

3770 

269 

1671700 

BT 

Sept.  5 

2 

1 

13 

20.5 

0 

0 

60 

390 

2360 

182 

559150 

CT 

Sept.  11 

3 

0 

14 

16.5 

0 

7 

210 

900 

8780 

627 

6614000 

DT 

Sept.  30 

Random  distribution. 

ET 

Oct.  21 

3 

Random  distribution. 

FT 

Oct.  31 

22 

Random  distribution. 

GT 

Nov.  10 

4 

0 

11 

17     ||  0   0   70 

390 

2180 

198 

500200 

H7 

Nov.  20 

14 

Random  distribution. 

IT 

Nov.  29 

9 

Random  distribution. 

JT 

Dec.  11 

2 

1 

12 

11.5   ||  0 

1 

130 

900 

4480 

337 

2272600 

KT 

Dec.  20 

3 

0 

6 

14     II  0 

1 

190 

900 

2220 

370 

1173600 

Lr 

Dec.  30 

19 

Random  distr 

bution. 

1917 

MT 

Jan.  10 

2 

0 

12 

18    |l  0 

5 

130 

900 

7810 

651 

6008700 

NT 

Jan.  19 

7 

Random  distr 

bution. 

Or 

Jan.  30 

4 

1 

6 

15    |l  .0 

0 

40 

180 

560 

93 

67400 

P7 

Feb.  7 

Random  distr 

butio 

n. 

Feb.  16 

2 

0 

9 

9.8 

0 

0 

120 

460 

1895 

211 

509025 

RT 

Feb.  24 

3 

o 

14 

12 

0 

2 

90 

900 

3730 

266 

2003100 

ST 

Mar.  4 

2 

0 

15 

11 

0 

3 

100 

900 

5020 

335 

2947000 

TT 

Mar  13 

3 

o 

12 

13 

0 

2 

70 

900 

3946 

329 

2332716 

UT 

Mar.  21 

3 

0 

16 

14 

0 

0 

60 

270 

2205 

138 

352225 

VT 

Mar.  29 

2 

0 

8 

14 

0 

0 

80 

170 

1010 

126 

134300 

W7 

April  9 

3 

o 

12 

12 

0 

0 

50 

160 

1310 

109 

161100 

XT 

April  18 

2 

1 

10 

15 

0 

0 

40 

525 

1671 

167 

466759 

YT 

April  25 

2 

0 

20 

14 

0 

0 

60 

185 

2210 

111 

265216 

•  Second  brood  from  this  mother. 

•>  The  M«  generation  was  taken  from  a  very  late  brood  of  the  L«  generation  which  was  used  in  con- 
ducting a  test  series. 


94 


SELECTION    IN    CLADOCERA    ON    THE    BASIS   OF 


TABLE  42. — Summary  of  selection  data  by  broods  for  Line  757  minus. 


1 

t 

1 

.2 

o 

.2 

a 

c 

i 

A 

1 

0 

i 

to 
.2 

I! 

0 

_a 

| 

S 

§ 

1 

a 

e 

8 

1  . 

1 

1 

1 

£  « 

.3  ~ 

a 

a  t 
.2 

1 

c 

0 

J 

I 

3.S 

* 

.2 

1 

J° 
?•§ 

1 

1 

I 

1 

1 

t 

. 

o 

I 

jj 

|js 

i: 

a 

a  _• 

s 

1 

•3 

i 

i| 

0 

3 

•8.1 

0  fe 

>» 

•s 

|| 

0  g 

—  •  C8 

I| 
•a  8 

iximu 
iconda 

a  i 

I 

a 

1 

=  8 

s  a 

• 

o 

Q1 

1 

3* 

6 

fc 

Is3 

*£ 

»5 

S  " 

£s 

CO  ** 

1912 

A 

Nov.  15 

1 

0 

5 

18 

.  0 

5 

900 

900 

4500 

900 

4050000 

B 

Nov.  27 

2 

0 

3 

16 

0 

3 

900 

900 

2700 

900 

2430000 

C 

Dec.  9 

2 

0  or  1 

6 

13.5 

0 

4 

245 

900 

4130 

688 

3381250 

D 

Deo.  20 

2 

0 

13 

19 

0 

11 

190 

900 

10615 

817 

9221725 

E 

Dec.  30 

2 

Oor  1 

14 

16 

0 

8 

260 

900 

10090 

781 

8078950 

1913 

F 

Jan.  8 

2 

0 

9 

17.5 

0 

5 

65 

900 

5270 

586 

4255950 

G 

Jan.  16 

1 

0 

2 

16 

0 

2 

900 

900 

1800 

900 

1620000 

H 

Jan.  24 

2 

0 

6 

15.5 

0 

2 

184 

900 

3975 

663 

2959925 

I 

Feb.  3 

3 

Oor  1 

2 

14.2 

0 

2 

900 

900 

1800 

900 

1620000 

J 

Feb.  12 

2 

0 

12 

13 

0 

11 

320 

900 

10220 

852 

9012400 

K 

Feb.  21 

2 

0 

18 

17 

0 

16 

290 

900 

15120 

840 

13229000 

L 

Mar.  3 

1 

Oor  1 

9 

11.8 

0 

7 

250 

900 

7105 

789 

6040525 

M 

Mar.  12 

2 

0 

9 

13.5 

0 

6 

250 

900 

6570 

730 

5365700 

N 

Mar.  21 

5 

0 

8 

0 

8 

900 

900 

7200 

900 

6480000 

O 

April  5 

2 

0 

16 

19.9 

0 

14 

555 

900 

13825 

864 

12096925 

P 

April  14 

4 

Oor  1 

7 

18.5 

0 

4 

372 

900 

5082 

723. 

4005684 

April  21 

2 

Oor  1 

10 

13.4 

0 

10 

900 

900 

9000 

900 

8100000 

April  28 

3 

Oor  t 

9 

17.6 

0 

9 

900 

900 

8100 

900 

7290000 

8 

May  7 

2 

0 

7 

17 

0 

7 

900 

900 

6300 

900 

5670000 

T 

May  17 

4 

0 

14 

16.2 

0 

14 

900 

900 

12600 

900 

11340000 

U 

June  4 

5 

0 

19 

19.2 

1 

17 

540 

900 

16550 

876 

14565700 

V 

June  13 

3 

0 

6 

17.5 

0 

6 

900 

900 

5400 

900 

4860000 

W 

June  20 

2 

0 

9 

19.5 

0 

9 

900 

900 

8100 

900 

7290000 

X 

June  26 

3 

0 

16 

20.6 

0 

16 

900 

900 

14400 

900 

12960000 

Y 

July  3 

3 

0 

7 

22.3 

0 

7 

900 

900 

6300 

900 

5670000 

Z 

July  10 

4 

0 

11 

22 

0 

11 

900 

900 

9900 

900 

8910000 

Z» 

July  14 

3 

3 

5 

22 

0 

5 

900 

900 

4500 

900 

4050000 

At 

July  23 

4 

0 

3 

22 

0 

3 

900 

900 

2700 

900 

2430000 

Bt 

July  29 

2 

0 

6 

22.5 

0 

5 

600 

900 

5100 

850 

4410000 

Ci 

Aug.  5 

4 

0 

6 

21.5 

0 

6 

900 

900 

5400 

900 

4860000 

Di 

Aug.  13 

5 

0 

4 

20.7 

0 

4 

900 

900 

3600 

900 

3240000 

E, 

Aug.  19 

7 

0 

12 

23 

0 

12 

900 

900 

10800 

900 

9720000 

Fi 

Aug.  26 

4 

0 

8 

20 

1 

7 

840 

900 

7140 

893 

6375600 

Gi 

Sept.  1 

10 

0 

17 

21.6 

0 

17 

900 

900 

15300 

900 

13770000 

Ht 

Sept.  8 

1 

2 

13 

22 

0 

13 

900 

900 

11700 

900 

10530000 

Ii 

Sept.  16 

11 

0 

11 

13.9 

0 

11 

900 

900 

9900 

900 

8910000 

fe 

Sept.  22 
Sept.  29 

10 
11 

0 
0 

15 
17 

20 
17.1 

0 
0 

6 

17 

167 
900 

900 
900 

8720 
15300 

681 
900 

6326504 
13770000 

Li 

Oct.  7 

4 

0 

6 

19.8 

0 

6 

900 

900 

5400 

900 

4860000 

Mi 

Oct.  14 

10 

0 

6 

15.1 

1 

5 

410 

900 

4910 

818 

4218100 

Ni 

Oct.  21 

3 

0 

12 

18.4 

0 

9 

168 

900 

9843 

820 

8567649 

0, 

Oct.  29 

4 

0 

8 

19.1 

0 

8 

900 

900 

7200 

900 

6480000 

Pi 

Nov.  8 

11 

0 

6 

17.7 

0 

3 

242 

900 

3830 

638 

2942004 

Q> 

Nov.  17 

11 

0 

13 

13.6 

0 

3 

255 

900 

6454 

497 

3915506 

Ri 

Nov.  28 

11 

0 

15 

15.5 

0 

13 

305 

900 

12347 

823 

10739989 

81 

Dec.  6 

11 

0 

11 

14.2 

0 

11 

900 

900 

9900 

900 

8910000 

T, 

Dec.  15 

12 

0 

12 

12.2 

0 

12 

900 

900 

10800 

900 

9720000 

Ut 

Dec.  24 

10 

0 

13 

10.7 

0 

10 

424 

900 

10534 

810 

8917876 

1914 

V, 

Jan.  2 

3 

0 

10 

9.1 

0 

10 

900 

900 

9000 

900 

8100000 

w, 

Jan.  10 

1 

0 

11 

13.9 

0 

10 

628 

900 

9628 

875 

8494384 

X, 

Jan.  21 

5 

1 

11 

14.3 

0 

10 

378 

900 

9378 

853 

8242884 

Yi 

Jan.  27 

2 

0 

7 

14 

0 

7 

900 

900 

6300 

900 

5670000 

Zi 

Feb.  9 

1 

0 

1 

9.8 

0 

1 

900 

900 

900 

900 

810000 

Ai 

Feb.  21 

Random  distribution. 

B, 

Mar.  2 

4 

1 

13 

16.2 

1 

1 

146 

900 

4907 

S78 

2538887 

Ci 

Mar.  11 

1 

0 

10 

22 

0 

10 

900 

900 

9000 

900 

8100000 

D, 

Mar.  20 

4 

0 

9 

18.3 

0 

9 

900 

900 

8100 

900 

7290000 

Ei 

Mar.  28 

4 

0 

17 

19 

0 

12 

80 

900 

12530 

737 

10623900 

Fi 

April  6 

3 

0 

9 

17.2 

0 

7 

300 

900 

6930 

770 

5868900 

Gi 

April  15 

6 

0 

18 

19.7 

0 

13 

80 

900 

13490 

749 

11352300 

H," 

May  7 

6 

0 

4 

20.7 

0 

3 

735 

900 

3435 

859 

2970225 

b 

May  18 

1 

0 

3 

19.1 

0 

3 

900 

900 

2700 

900 

2430000 

Ji 

May  26 

6 

0 

10 

21.2 

0 

9 

110 

900 

8210 

821 

7302100 

K, 

June  2 

1 

0 

11 

19.1 

0 

9 

370 

900 

8847 

804 

7560029 

u 

June  13 

3 

0 

15 

20.7 

0 

10 

175 

900 

10424 

695 

8645366 

•  Selection  was  repeated  from  a  later  brood  because  of  the  loss  of  the  earlier  brood. 
b  Death  losses  in  this  strain  made  this  interval  between  generations  unusually  long. 


A    PHYSIOLOGICAL   CHAHACTER. 


95 


TABLE  42. — Summary  of  selection  data  by  broods  for  Line  757  minus — Continued. 


J 

• 

1 

i 

S 

o 

.5 

a 

| 

A 

1 

-3 

1 

J? 

11 

| 

0 

^ 

'•g 

1 

•g 

I 

1 

1 

1 

!! 

I 

| 

.3 

49 

•5 

§ 

I 

•1 

c 

c 

£f 

I? 

1 

•j 

•J 

1 

i 

c-S 

1 

£ 

1 

11 

£ 

1 

g 

| 

1 

"*! 

$ 

|lj 

1 

—  *£ 

li 

B* 

S  . 

^  . 

1 

B 

|| 

*0 

C 

7  E 

Si 

6 

N 

11 

c  i 

|| 

If 

l| 

1 

§1 

0 

°" 

K 

5  & 

z 

r~  *» 

fc73 

25* 

IS 

S  - 

m  S 

S 

3  w 

1914 

M, 

June  22 

4 

0 

7 

19.2 

0 

4 

150 

900 

4412 

630 

3482344 

Ni 

June  29 

2 

0 

7 

19.3 

0 

0 

IIS 

245 

1191 

170 

214923 

0, 

July  7 

5 

0 

10 

IS  3 

1 

7 

3SO 

9Ov 

8101 

810 

6850241 

Pi 

July  14 

3 

0 

13 

21.6 

0 

9 

420 

900 

105SO 

814 

8881400 

Qj 

Julv  20 

3 

0 

7 

21 

0 

2 

225 

900 

41SO 

597 

2880150 

Ri 

July  27 

4 

0 

4 

21.7 

0 

1 

SO 

900 

1270 

318 

859700 

Si 

Aug.  5 

3 

0 

4 

21.4 

0 

3 

230 

900 

2930 

733 

2482900 

Ti 

Aug.  13 

2 

0 

5 

0 

1 

290 

900 

2553 

511 

1547969 

Di 

Aug.  21 

3 

0 

10 

23.1 

0 

7 

300 

900 

7551 

755 

6213501 

V, 

Am: 

3 

0 

7 

21 

0 

3 

150 

900 

4333 

619 

3205749 

w, 

Sept.  2 

1 

0 

13 

22.6 

1 

9 

160 

900 

9603 

739 

8160549 

Xs 

Sept.  8 

4 

0 

15 

18 

0 

1 

85 

900 

4145 

276 

1789001 

Yi 

Sept.  19 

11 

0 

13 

18 

0 

1 

110 

900 

3S25 

294 

1896125 

z, 

Sept.  25 

2 

0 

11 

19 

0 

9 

370 

900 

$975 

816 

7681925 

A4 

Oct.   6 

3 

1 

13 

19.8 

0 

12 

175 

900 

10975 

844 

9750625 

B4 

Oct.  13 

3 

0 

18 

19.1 

0 

17 

105 

900 

15405 

S56 

13781025 

C, 

Oct.  21 

11 

0 

16 

18.9 

0 

16 

900 

900 

1441X1 

900 

12960000 

Di 

Oct.  29 

4 

0 

15 

18 

0 

15 

900 

900 

13500 

900 

12150000 

E« 

Nov.  6 

2 

0 

19 

17.9 

0 

14 

260 

900 

15761 

8SO 

13536465 

F« 

Nov.  13 

2 

0 

16 

17.9 

0 

16 

900 

900 

H400 

900 

12960000 

G, 

Nov.  20 

3 

0 

14 

20.8 

0 

14 

900 

900 

12600 

900 

11340000 

H4 

Nov.  2S 

11 

0 

10 

18 

0 

10 

900 

900 

9000 

900 

8100000 

L 

Doc.  7 

3 

1 

8 

19.8 

0 

7 

215 

900 

6515 

814 

5716225 

Ji 

Dec.  16 

3 

0 

1 

17 

0 

0 

310 

310 

310 

310 

96100 

K« 

Dec.  24 

2 

0 

8 

18 

0 

8 

900 

900 

7200 

900 

6480000 

1915 

L, 

Jan.   1 

11 

0 

12 

17.9 

0 

9 

204 

900 

9014 

751 

7087716 

M« 

Jan.  8 

3 

0 

11 

19.5 

0 

10 

S25 

900 

9S25 

89S 

S7S0625 

N4 

Jan.  15 

2 

0 

15 

19.9 

0 

15 

900 

900 

13500 

900 

12150000 

04 

Jan.  25 

3 

1 

15 

17.2 

0 

14 

312 

900 

12912 

861 

11437344 

P« 

Feb.  3 

4 

0 

16 

15.3 

1 

5 

57 

900 

S025 

502 

5341163 

Q« 

Feb.  13 

4 

0 

10 

16.5 

0 

2 

210 

POO 

4145 

415 

2373475 

R4 

Feb.  22 

3 

0 

11 

19.6 

1 

4 

145 

900 

5390 

490 

3725000 

SU 

Mar.  2 

1 

0 

10 

20 

0 

0 

150 

555 

2370 

237 

698150 

T4 

Mar.  10 

7 

0 

15 

20 

1 

7 

125 

900 

9076 

605 

6748964 

U4 

Mar.  19 

2 

0 

7 

18.2 

0 

1 

35S 

900 

3547 

507 

2006369 

V, 

Mar.  30 

2 

0 

10 

16.8 

0 

1 

210 

900 

5025 

503 

3112775 

W4 

April  7 

3 

0 

12 

17.5 

0 

10 

100 

900 

9345 

779 

8170025 

X. 

April  15 

3 

0 

16 

17.3 

0 

12 

ISO 

900 

12172 

761 

10295594 

Y, 

•l 

April  23 
Mav  1 

3 

2 

0 
0 

14 

15 

18.7 
19.4 

0 
0 

14 
9 

900 
2S2 

900 

900 

12600 
11507 

900 
767 

11340000 
9479349 

At 

May  10 

6 

0 

12 

19.2 

0 

12 

900 

900 

10800 

900 

9720000 

BI 

v  21 

3 

0 

13 

18.3 

0 

0 

135 

475 

3523 

271 

1104857 

C. 

May  31 

3 

0 

14 

15.5 

0 

13 

620 

900 

12320 

880 

10914400 

Di 

June  9 

4 

0 

12 

18.4 

0 

10 

125 

900 

9555 

796 

8300525 

E» 

June  16 

3 

0 

20 

21 

0 

6 

120 

900 

9S55 

493 

6737025 

F» 

June  24 

5 

0 

7 

19 

0 

0 

SO 

2SS 

1118 

160 

203844 

G, 

July  3 

3 

0 

8 

21.2 

0 

2 

250 

900 

5152 

644 

3S60034 

H» 

Julv  10 

3 

0 

7 

21.2 

0 

7 

900 

900 

631X1 

900 

5670000 

Is 

July  17 

3 

0 

9 

22.1 

0 

8 

110 

900 

7310 

812 

6492100 

J» 

July  25 

Random  distribution. 

K. 

Aug.  2 

4 

1 

1 

26 

0 

5 

184 

900 

5484 

686 

4407906 

L. 

Aug.  10 

4 

0 

3 

22.6 

0 

2 

470 

900 

2270 

757 

1  $40900 

M. 

Aug.  16 

3 

0 

8 

24.1 

0 

8 

900 

900 

7200 

900 

6480000 

Ni 

Aug.  23 

3 

0 

10 

21.2 

3 

1 

150 

900 

5340 

534 

3433800 

0, 

Aug.  31 

2 

0 

9 

18.9 

0 

9 

900 

900 

8100 

900 

7290000 

Pi 

Sept.  8 

5 

0 

6 

22.2 

0 

5 

460 

900 

4960 

S27 

4261600 

Q> 

Sept.  14 

6 

0 

10 

21.9 

0 

8 

150 

900 

7550 

755 

6542500 

R. 

Sept.  20 

5 

1 

13 

20.1 

0 

11 

270 

900 

10440 

803 

9055800 

S* 

Oct.   4 

4 

0 

7 

17.8 

0 

7 

900 

900 

6300 

900 

5670000 

T* 

Oct.  13 

4 

0 

17 

17.8 

0 

14 

510 

900 

14255 

839 

12255625 

Ui 

Oct.  21 

3 

0 

13 

18.8 

0 

13 

900 

900 

11700 

900 

10530000 

Nov.  1 

5 

1 

13 

17 

1 

8 

90 

900 

9215 

709 

7685425 

\V4 

Nov.  9 

4 

0 

20 

15.5 

1 

6 

175 

900 

107SO 

539 

7539150 

X* 

Nov.  19 

5 

0 

18 

16 

0 

12 

150 

900 

12510 

695 

10311300 

YI 

Nov.  29 

5 

0 

23 

14.5 

0 

16 

160 

900 

17620 

766 

14  $4  7050 

z. 

Dec.  9 

5 

0 

IS 

14 

0 

11 

150 

900 

12225 

679 

9752925 

A. 

Doc.  18 

7 

0 

19 

16.5 

0 

19 

900 

900 

17100 

900 

15390000 

B. 

Dec.  28 

4 

1 

17 

14.2 

0 

17 

900 

900 

13900 

900 

13770000 

SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 


TABLE  42. — Summary  of  selection  data  by  broods  for  Line  757  minus — Continued. 


a 

A 

3 

1 

2 

a 

.S 

a 

4 

a 

! 

a 

OJ 

0 

• 
1 

i 

a 
1 

1 

11 

1 

1 

S 
S 
so 

a 

1 

a 

i 

2 

"a 

g 

a 

£ 

m  5 

g 

§ 

§ 

'v 

0 

1 

S  . 

s!a 

.s 

9 
a 

I 

£ 

ji 

•3 

1 

.2 

1 

8 

o-g 

g 

1 

§JS 

.2  g 

S  • 

8  « 

J  • 

B 

1 

08 

*l 

•s 

o| 

0 

Id 

•gl 

"s-S 

3  >§ 
Ii 

si 

°1 

C 

°  8 

i 

^  09 

oa  -C 

=j 

i! 

6 

s  g 

6  -~ 

6  S 

'3  o 

11 

g  8 

i 

• 

II 

0 

W 

*  ft 

* 

*• 

S'0 

*  * 

ii  • 

S  " 

«  B 

S 

1916 

c, 

Jan.  11 

6 

1 

11 

18 

0 

11 

900 

900 

9900 

900 

8910000 

D, 

Jan.  23 

3 

2 

10 

19 

0 

10 

900 

900 

9000 

900 

8100000 

Bi 

Feb.  1 

5 

0 

6 

11.2 

0 

6 

900 

900 

5400 

900 

4860000 

F, 

Feb.  10 

8 

0 

18 

13.2 

0 

18 

900 

900 

16200 

900 

14580000 

G. 

Feb.  19 

3 

0 

14 

15.5 

0 

4 

140 

900 

6695 

478 

4428675 

Hi 

Mar.  1 

4 

0 

13 

15 

0 

11 

420 

900 

10920 

840 

9446400 

L 

Mar.  11 

4 

0 

12 

16.4 

0 

7 

90 

900 

7325 

610 

5909325 

2 

Mar.  21 

3 

0 

13 

„  15.8 

0 

13 

900 

900 

11700 

900 

10530000 

K« 

Mar.  31 

9 

0 

16 

15.2 

0 

16 

900 

900 

14400 

900 

12960000 

Le 

April  18 

•71 

M, 

May  2 

3 

0 

15 

'l6.5 

6 

"l3 

220 

900 

12210 

814 

10662500 

May  13 

10 

0 

18 

16 

0 

18 

900 

900 

16200 

900 

14580000 

0,' 

May  24 

4 

0 

22 

18.3 

0 

21 

150 

900 

19050 

866 

17032500 

Pe 

June  1 

2 

0 

13 

18.5 

0 

.  7 

65 

900 

8435 

649 

6765375 

June  12 

1 

18 

17 

0 

17 

240 

900 

15540 

863 

13827600 

R« 

June  22 

0 

12 

18.6 

0 

0 

60 

450 

3305 

275 

1141325 

ft 

June  29 

1 

19 

19 

0 

11 

135 

900 

11825 

622 

9453525 

T. 

July  6 

0 

11 

19.2 

0 

11 

900 

900 

9900 

900 

8910000 

U, 

July  14 

1 

7 

21 

0 

6 

175 

900 

5575 

796 

4890625 

V, 

July  22 

0 

6 

21.1 

0 

4 

80 

900 

3770 

628 

3254900 

July  28 

o 

11 

21 

o 

7 

X,' 

Aug.  4 

ii 

0 

3 

19.3 

1 

2 

420 

900 

2220 

740 

1796400 

Y, 

Aug.  11 

2 

0 

7 

19 

0 

3 

50 

900 

4110 

587 

3169150 

Z. 

Aug.  17 

3 

0 

8 

20.6 

0 

7 

260 

900 

6560 

820 

5737600 

AT 

Aug.  24 

9 

1 

8 

21.3 

0 

8 

900 

900 

7200 

900 

6480000 

BT 

Aug.  30 

1 

0 

9 

17 

0 

4 

360 

900 

6470 

719 

4984500 

CT 

Sept.  6 

3 

0 

12 

19.5 

0 

9 

220 

900 

9130 

761 

7702900 

D7 

Sept.  13 

1 

0 

10 

17 

0 

8 

330 

900 

7880 

780 

6711400 

ET 

Oct.  20 

Random  distribution. 

FT 

Oct.  29 

21 

Random  distributio 

n. 

GT 

Nov.  8 

"2" 

0 

16 

16 

0 

14 

300 

900 

13200 

825 

11520000 

HT 

Nov.  17 

2 

0 

14 

13 

0 

12 

64 

900 

11014 

787 

9746596 

IT 

Nov.  25 

3 

0 

8 

10.5 

0 

8 

900 

900 

7200 

900 

6480000 

JT 

Dec.  7 

2 

0 

7 

13.2 

0 

1 

50 

900 

2160 

309 

1208800 

K7 

Dec.  16 

2 

0 

20 

11.5 

0 

8 

60 

900 

8990 

450 

6774100 

Lr 

Dec.  25 

Rand 

)in  distrit 

ution. 

1917 

MT 

Jan.  4 

2 

0 

6 

12.5 

0 

0 

100 

210 

795 

133 

113725 

NT 

Jan.  13 

4 

0 

10 

11.2 

0 

2 

80 

900 

2670 

267 

1717750 

OT 

Jan.  20 

4 

0 

14 

12 

0 

0 

100 

480 

2970 

212 

735700 

PT 

Jan.  30 

4 

1 

26 

15.5 

0 

21 

380 

900 

21150 

814 

18094900 

Or 

Feb.  10 

3 

1 

4 

10.5 

0 

2 

450 

900 

2830 

708 

2158900 

RT 

Feb.  19 

3 

1 

11 

11 

0 

2 

150 

900 

4560 

415 

2699800 

ST 

Feb.  28 

2 

1 

7 

10.5 

0 

3 

110 

900 

3000 

429 

1996200 

TT 

Mar.  9 

3 

0 

9 

17  t 

0 

7 

50 

900 

6440 

716 

5680600 

UT 

Mar.  17 

Rand 

m  distribution. 

VT 

Mar.  26 

3 

1 

10 

15.5 

0 

7 

110 

900 

7140 

714 

5948600 

WT 

April  3 

2 

0 

16 

15.2 

0 

9 

130 

900 

9920 

620 

7822000 

XT 

April  11 

3 

0 

15 

13.5 

0 

4 

270 

900 

8220 

548 

5382400 

YT 

April  21 

11 

0 

14 

18 

0 

10 

25 

900 

9419 

673 

8165063 

•  This  selection  made  from  a  fourth  brood  of  71  individuals  constituting  a  part  of  a  test  series.    Data  Dot 
included  here  as  not  strictly  comparable  to  other  data  of  this  table. 

b  Selection  made,  but  time  of  beginning  of  experiment  inadvertently  omitted. 

the  plus  was  the  more  reactive  during  every  month  except  for  4 
months  during  the  period  of  irregularity  in  reactiveness  in  1914  and 
1915  (see  pages  115-116  and  table  44). 

Averages  of  the  data  by  two-month  periods  (tables  41  and  42), 
such  as  are  given  for  Lines  695,  689,  711,  and  719,  serve  still  further 
to  compensate  for  temporary  irregularities  in  reactiveness  and  are 
used  in  the  analysis  of  the  data  for  line  757  and  as  a  basis  for  the 
curves  given  in  figure  18e. 


A    PHYSIOLOGICAL   CHARACTER.  97 

DETAILED  ANALYSIS  OP  DATA  FOR  LINE  757. 

The  curve  of  the  reaction-times  for  Line  757  (figure  18s)  begins 
with  the  December  1912-January  1913  period,  though  it  includes  a 
few  reaction-time  records  obtained  in  November.1  The  means  for 
this  period  are  identical,  but  in  the  following  period  there  is  a  slight 
divergence  (the  minus  strain  being  the  less  reactive)  followed  by 
irregular  but  in  general  increased  divergences  in  the  later  two-month 
periods,  so  that  the  course  of  the  curves  is  from  the  start  suggestive 
of  an  effect  of  selection. 

There  is  one  period  of  two  months  (June-July  1914)  for  which 
the  minus  strain  had  the  lower  reaction-time  and  the  difference  was 
fairly  large  (132  seconds;  table  44).  In  3  other  two-month  periods 
the  minus  strains  had  mean  reaction-times  only  2,  20,  and  48  seconds 
higher  than  those  for  the  plus  strains  for  the  corresponding  periods. 
But  in  all  the  other  two-month  periods  of  the  experiment  after  the 
first  5  months  of  selection  the  mean  reaction-time  for  the  minus 
strain  was  from  55  to  629  seconds  higher  than  that  for  the  plus  strain 

.e-e e ^ o ;  i  *; :  •  3.  i ' 

48060?      10°  O      O     |0     O      O      O 

7    o    o    o    o    o    g    '3    3          °    6     ' 

12100 

ll-l         4-5        8-9         12-1        4-5        8-9        12-1        4-5        8-9        12-1        4-5        8-9        12-1  4 

1913  1914  1915  1916  1917 

FIGURE  16. — Line  757.  Relative  rates  of  descent  of  the  two  strains.  (See  figure  2 A  for  description.) 

(tables  43  and  44).  Hence  the  mean  reaction- time  differences  by 
two-month  periods  are  nearly  all  in  the  direction  sought  in  selection, 
and  most  of  them  are  differences  of  statistical  significance.  Calculated 
by  longer  periods,  all  the  differences  are  of  large  statistical  value, 
the  minus  strains  being  the  less  reactive.  This  will  be  made  clear 
in  the  following  detailed  analysis. 

REACTION-TIME  MEANS  COMPARED  BY  LONGER  PERIODS. 

For  the  first  9  months  of  the  experiment  (November  1912- July 
1913)  the  means  for  the  plus  and  minus  strains  (29  generations  and 
320  individual  reaction-time  records,  and  28  generations  and  261 

1  Data  for  two  abnormally  small  broods  of  each  strain  were  obtained  during  November. 
These  data  consist  of  the  reaction-times  of  only  6  individuals  of  the  plus  strain  and  8  individuals 
of  the  minus  strain,  and  of  course  give  averages  of  little  value,  so  that  it  seems  best  to  combine 
these  data  with  the  data  for  the  following  two  months  in  constructing  the  reaction- time  curves. 
The  means  (see  tables  43  and  44)  for  the  two  strains  for  November  alone  are  685  seconds  for 
the  plus  strain  and  900  seconds  for  the  minus  strain.  All  8  of  the  minus  strain  and  4  of  the  6 
of  the  plus  strain  were  "over-time"  individuals.  These  meager  data  have  little  significance, 
except  to  indicate  that  both  strains  were  very  slightly  reactive  at  the  beginning  of  the  experi- 
ment. For  the  December  1912-January  1913  period  alone  (excluding  the  few  November 
records)  the  means  are  747  and  718  seconds.  Hence  the  curves  with  the  data  for  November 
separate  would  start  with  the  minus  strain,  the  less  reactive  by  215  seconds,  to  be  followed  by 
the  December-January  period,  when  the  plus  was  the  less  reactive  by  29  seconds.  In  tables 
43  and  44  the  data  are  presented  with  the  November  data  separate  from  those  for  the  December- 
January  period. 


98 


SELECTION   IN   CLADOCERA    ON   THE    BASIS   OF 


individual  reaction-time  records,  respectively)  were:  for  the  plus 
strain  788.3  seconds,  and  for  the  minus  strains  838.9  seconds  (table 
45).  The  difference  (-50.6 ±11. 18  seconds)  is  4.53  times  the 
statistical  probable  error.  These  data  include  those  for  the  5  months 
at  the  beginning  of  the  experiment,  when  selection  was  only  slightly 
if  at  all  effective.  For  this  nine-month  period  there  are  9  same-day 
broods,  containing  114  and  99  individuals,  with  the  reaction-time  for 
the  minus  strain  averaging  58  ±16. 24  seconds  greater  than  that 

TABLE  43. — Selection  data  summarized  by  two-month  periods  for  Line  757  plus. 


i 

1 

*fe 

S« 

S  | 

.3 

2 

2 

s-a 

| 

1 

.J 

i 

j 

to 

a 

|l 

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198.86 

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30-38 

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1 

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301.07 

18.54 

Oct.-Nov.  1913.  .  . 

39-45 

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309 

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46-52 

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0 

33 

279 

811 

48045 

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309.38 

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Feb.-Mar.  1914... 

53-59 

7 

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14.9 

8.3 

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2 

57 

348 

900 

67229 

646 

296.25 

19.59 

Apr.-May    1914... 

60-65 

6 

72 

12.0 

8.2 

.46 

0 

46 

423 

900 

51828 

720 

266  .  32 

21.17 

June-  July   1914... 

66-73 

8 

76 

9.9 

7.6 

.30 

0 

61 

395 

900 

60373 

794 

227.95 

17.64 

Aug.-Sept.  1914... 

74-81 

7 

80 

13.3 

7.1 

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2 

29 

241 

791 

44890 

561 

298.18 

22.47 

Oct.-Nov.  1914.  .  . 

82-89 

8 

119 

15.0 

8.3 

.81 

2 

69 

243 

859 

78240 

657 

303.13 

18.74 

Dec.  1914-Jan.  1915 

90-96 

7 

69 

10.2 

8.2 

.24 

44 

493 

900 

49288 

714 

269.78 

21.91 

Feb.-Mar.  1915... 

97-102 

6 

66 

11.4 

9.5 

.20 

0 

16 

121 

742 

27182 

412 

305.76 

25.38 

Apr.-May   1915... 

103-108 

5 

53 

11.0 

9.7 

.13 

1 

24 

261 

863 

34563 

652 

281.37 

26.07 

June-July    1915... 

109-116 

8 

85 

9.9 

7.6 

.30 

0 

34 

304 

815 

51316 

604 

281.06 

20.56 

Aug.-Sept.  1915... 

117-123 

7 

60 

10.0 

7.4 

.35 

0 

26 

187 

900 

35235 

587 

323.40 

28.16 

Oct.-Nov.  1915.  .  . 

124-130 

7 

132 

17.8 

9.2 

.93 

5 

31 

132 

763 

57260 

434 

270.87 

15.90 

Dec.  1915-Jan.  1916 

131-136 

6 

95 

17.8 

9.8 

.82 

9 

7 

255 

689 

29685 

312 

200.72 

13.89 

Feb.-Mar.  1916  .  .  . 

137-142 

6 

99 

15.2 

10.3 

.48 

1 

42 

251 

847 

51895 

524 

336.60 

22.82 

Apr-May   1916... 

143-145 

3 

45 

17.8 

9.8 

.82 

0 

22 

190 

900 

26125 

581 

255.24 

25.66 

June-July   1916... 

146-152 

7 

74 

10.6 

8.3 

.28 

0 

15 

143 

805 

31188 

421 

276.37 

21.67 

Aug.-Sept.  1916... 

153-160 

6 

82 

13.7 

6.9 

.99 

0 

23 

109 

815 

36460 

445 

309.23 

23.03 

Oct.-Nov.  1916  .  .  . 

161-165 

1 

11 

12.8 

11.0 

.16 

0 

0 

70 

390 

2180 

198 

78.92 

16.05 

Dec.  1916-Jan.  1917 

166-171 

4 

36 

10.4 

10.0 

.04 

0 

7 

123 

720 

15070 

419 

298.80 

33.59 

Feb.-Mar.  1917... 

172-178 

6 

74 

12.3 

8.8 

.40 

0 

7 

87 

600 

17806 

241 

232.34 

18.22 

Apr.  1917  

179-181 

3 

42 

14.0 

9.2 

.52 

0 

0 

50 

290 

5191 

124 

77.53 

8.07 

for  the  plus  strain.  This  difference  is  3.59  times  its  probable  error. 
Only  one  of  these  minus  same-day  broods  had  a  mean  reaction-time 
lower  than  that  for  the  corresponding  plus  brood. 

For  the  year  August  1913- July  1914  (44  generations  and  504 
individual  reaction-time  records,  and  42  generations  and  412  indi- 
viduals in  the  plus  and  minus  strains,  respectively),  the  mean  re- 
action-times were  683.7  seconds  and  783.0  seconds  (table  45).  The 
difference  in  mean  reaction-time  was  —99.3  ±11. 77  seconds,  or  8.44 
times  the  probable  error.  The  1 1  same-day  broods  for  this  period 
(145  and  123  individuals,  table  46)  gave  a  difference  in  reaction- 


A   PHYSIOLOGICAL   CHARACTER. 


99 


time  of  — 135.0  ±21. 74  seconds.  Because  of  the  smaller  total 
numbers  of  individuals  of  the  same-day  broods,  the  statistical  prob- 
able error  was  relatively  large,  but  the  difference  was  still  6.31  times 
the  probable  error.  In  only  one  of  these  same-day  broods  was  the 
reaction-time  for  the  brood  of  the  minus  strain  smaller  than  that 
for  the  plus  strain.  However,  during  one  period,  the  last  two-month 
period  of  this  year,  the  minus  strain  had  a  reaction-time  132  seconds 
lower  than  that  for  the  plus  strain.  This  was  a  period  of  rather 

TABLE  44. — Selection  data  summarized  by  two-month  periods  for  Line  757  minus. 


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22-28 

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7.4 

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1 

79 

827 

900 

72950 

890 

55.42 

4.13 

-  77 

15.56 

4.94 

Aug.-Sept.  1913... 

29-37 

9 

103 

11.4 

6.9 

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1 

93 

812 

900 

87860 

853 

157.60 

10.47 

-170 

21.29 

7.98 

Oct.-Nov.  1913  ... 

38-44 

7 

66 

9.4 

8.3 

.13 

1 

47 

454 

900 

49984 

757 

242.21 

20.11 

-  55 

32.67 

1.68 

Dec.  1913-Jan.  1914 

45-51 

7 

75 

10.7 

8.3 

.29 

0 

70 

719 

900 

65540 

874 

100.95 

7.86 

-288 

24.35 

11.82 

Feb.-Mar.  1914.  .. 

52-57 

5 

50 

10.0 

9.1 

.10 

1 

33 

585 

900 

35437 

709 

291.54 

27.81 

-  63 

34.02 

1.85 

Apr-May  1914.  .  . 

58-62 

5 

44 

8.3 

8.7 

.95 

0 

35 

425 

900 

34765 

790 

236.26 

24.02 

-  70 

32.02 

2.18 

June-July  1914  ... 

63-70 

8 

74 

7.7 

7.5 

.03 

1 

42 

240 

818 

49005 

662 

306.09 

24.00 

+  132 

29.79 

4.43 

Aug.-Sept.  1914... 

71-78 

8 

78 

11.8 

7.6 

.55 

1 

34 

212 

900 

43915 

563 

325.30 

24.84 

-     2 

33.49 

0.05 

Oot.-Nov.  1914  .  .  . 

79-86 

8 

121 

16.0 

8.1 

.98 

0 

114 

630 

900 

106041 

876 

117.19 

7.19 

-219 

20.07 

10.91 

Dec.  1914-Jan.  1915 

87-93 

7 

70 

10.3 

8.1 

.27 

0 

63 

524 

816 

59276 

847 

171.25 

13.81 

-133 

25.89 

5.13 

Feb.-Mar.  1915  .  .  . 

94-100 

7 

79 

11.7 

9.3 

.26 

3 

20 

179 

851 

37578 

476 

278.70 

21.15 

-  64 

33.04 

1.93 

Apr.-May  1915.  .  . 

101-107 

7 

96 

12.9 

9.6 

.34 

0 

70 

445 

839 

72267 

753 

262.89 

18.10 

-101 

31.74 

3.18 

June-July  1915  ... 

108-114 

6 

63 

10.8 

7.8 

.38 

0 

33 

264 

798 

39290 

624 

327.67 

27.85 

-  20 

34.62 

0.57 

Aug.-Sept.  1915... 

11.5-122 

8 

67 

8.4 

7.1 

.18 

3 

49 

436 

900 

51344 

766 

243.39 

20.06 

-179 

34.57 

5.17 

Oct.-Nov.  1915  .  .  . 

123-129 

7 

111 

17.1 

9.7 

.76 

2 

76 

412 

900 

82380 

742 

263.55 

16.87 

-308 

23.18 

13.28 

Dec.  1915-Jan.  1916 

130-134 

5 

75 

16.0 

10.3 

1.55 

0 

68 

750 

900 

62125 

828 

244.04 

9.01 

-516 

23.54 

21.92 

Feb.-Mar.  1916  .  .  . 

135-141 

7 

92 

11.9 

10.2 

1.17 

0 

75 

607 

900 

72640 

790 

241.59 

6.99 

-266 

28.45 

9.34 

Apr-May  1916... 

142-145 

3 

55 

18.3 

11.0 

1.66 

0 

52 

423 

900 

47460 

863 

155.04 

4.10 

-282 

29.28 

9.63 

June-  July  1916  ... 

146-153 

7 

86 

12.8 

8.2 

1.56 

0 

56 

236 

836 

58350 

678 

317.39 

3.08 

-275 

31.66 

8.11 

Aug.-Sept.  1916..  . 

154-160 

7 

57 

8.1 

6.7 

1.21 

0 

41 

363 

900 

43570 

764 

240.07 

1.45 

-319 

31.47 

10.13 

Oct.-Nov.  1916  .  .  . 

161-165 

3 

38 

15.1 

9.0 

1.68 

0 

34 

421 

900 

31414 

827 

216.57 

23.70 

-629 

28.62 

21.97 

Dec.  1916-Jan.  1917 

166-172 

6 

83 

13.3 

8.9 

1.49 

0 

32 

128 

715 

38735 

467 

356.93 

26.42 

-  48 

42.74 

1.12 

Feb.-Mar.  1917... 

173-178 

5 

41 

8.2 

9.5 

.86 

0 

21 

174 

900 

23970 

585 

330.26 

34.79 

-344 

39.27 

8.75 

April,  1917  

179-181 

3 

45 

15.0 

8.8 

1.71 

0 

23 

142 

900 

27559 

612 

315.97 

31.77 

-488 

32.78 

14.88 

severe  losses  by  death  in  both  strains  and  particularly  in  the  plus 
strain.  In  general,  however,  there  has  been  no  obvious  relation 
between  losses  by  death  and  reaction-time,  and  it  is  doubtful  if  that 
was  an  influence  at  this  time. 

The  test  series  conducted  during  September  of  this  year-period 
contained  14  broods  of  each  strain,  322  individuals  of  the  plus  strain 
and  367  individuals  of  the  minus  strain.  The  means  were  409.0 
seconds  for  the  plus  strain  and  859.7  seconds  for  the  minus  strain 
(table  45) .  Thus  the  mean  for  the  minus  strain  was  more  than  twice 
that  for  the  plus  strain.  The  difference  ( -450.7  ±12.49  seconds) 


100 


SELECTION   IN    CLADOCERA   ON    THE   BASIS   OF 


was  36.08  times  its  probable  error.  In  every  case  in  this  test  series 
the  plus  brood  had  a  lower  reaction-time  than  the  corresponding 
minus  brood. 

For  the  year  August  1914-July  1915  (43  generations  and  472 
individuals,  and  44  generations  and  507  individuals,  respectively) 

TABLE  45. — Selection  summary  for  Line  757, 


1-d 

j 

5 

A 

| 

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d 

1 

6 

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8 

a 

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a 

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Time  period. 

Strain. 

0 

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do 

a  to 
£.2 

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1° 

| 

| 

• 

1 

1 

Is 

1 

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1 

1 

55  "g 

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1 

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d  8 

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02 

i 

gl 

I 

s3 

Nov.  16,  1912- 

(  Plus 

1-29 

29 

11.0 

320 

J 

247 

468 

900 

788.3 

226.8 

8.57 

July  81,  1913 

I  Minus 

1-28 

£8 

0.0 

501 

J 

030 

000 

535.0 

171.9 

7.15 

-50.0 

ii.15 

4.53 

Aug.  1,    1913- 

(  Plus  .  . 

30-73 

44 

11.5 

504 

3 

302 

357 

879 

683.7 

290.4 

8.72 

July  31,  1914 

I  Minus 

29-70 

41 

10.0 

412 

4 

320 

545 

554 

753.0 

238.1 

7.01 

-00!  S 

ii!77 

'5!  44 

34 

14 

23.0 

322 

7 

74 

112 

823 

409.0 

301.7 

11.34 

Sept.  1913..' 

!  Minus 

S3 

14 

26.2 

307 

330 

400 

000 

550.7 

149.1 

5.  25 

-450.7 

if.  40 

30.05 

Aug     1     1913— 

i   Plug 

30-42 

13 

12  1 

157 

1 

98 

376 

882 

691.2 

292.0 

15.72 

Oct.  31,  1913 

\  Minus 

29-41 

IS 

10.4 

135 

745 

000 

553.4 

156.4 

0.05 

-162.2 

15.10 

5.03 

Aug     1    1914— 

(     T^lllH 

74-116 

41 

11  5 

472 

6 

206 

282 

829 

604.8 

305.4 

9  48 

July  31,  1915 

I  Minus 

71-114 

43 

11.8 

507 

4 

334 

350 

554 

700.5 

290.5 

8.  70 

-102.0 

15.57 

7.03 

Aug    1     1915— 

i    Plug 

117-152 

36 

14  0 

505 

22 

143 

190 

811 

458.2 

294.7 

8.84 

July  31,  191b 

I  Minus 

115-163 

37 

12.9 

450 

5 

370 

407 

555 

770.2 

260.7 

7.05 

-312.0 

11.01 

50.50 

Test   series 

f   Plug  .  . 

143 

12 

54.3 

652 

0 

433 

172 

900 

697.0 

309.3 

8.17 

Apr.  1916... 

I  Minus 

1A9 

12 

50.4 

077 

0 

042 

000 

504-0 

156.  S 

4-05 

-167.6 

0.1* 

15.35 

Mar    1    1916- 

(Plus. 

140-145 

7 

16.0 

112 

1 

54 

259 

854 

564.3 

314.9 

20.08 

May31,  1916 

Minus 

138-145 

7 

15.0 

100 

0 

611 

000 

842.2 

186.9 

15.07 

-277.9 

53.43 

11.50 

Aug.    1,  1916- 

(  Plus.  . 

153-181 

20 

12.3 

245 

0 

37 

94 

632 

313.1 

280.2 

12.07 

May  1,  1917. 

I  Minus  . 

154-181 

24 

11.  0 

0 

151 

245 

554 

625.9 

333.7 

13.55 

-312.8 

15'.S7 

i7:6i 

TABLE  46. — Same-day  broods.    Summary  of  data  for  Line  757. 


S 

a 

o 

i 

* 

a 

j 

g 

« 

i 

K 
M 

'$ 

14 

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a 

8  . 

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1 

11 

1 

1 

i 

0 

1 

s  a 

i 

Qi 

J5 

h 

I 

o 

rl 

a" 

d 

I 

«  ^ 

Time  period. 

Strain. 

| 

d 

0      . 

individuals, 

negatively  i 
tals. 

individual; 
an  end  of  tl 

e  minimun 

1 

individual 

rd  deviatio 

ile  error  of  i 

nee  betweei 
nd  minus  s 

•3 

I 

i 

0 

11 

*o 

^•s 

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2  * 

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a  •- 

1 

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V    CO 

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s  ® 

d 

d 

d  ;g 

d  S 

>  -J 

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1 

to  ^ 

1 

fH  2 

125 

^ 

fc 

* 

^  M 

^^ 

^^ 

S** 

02 

(X. 

Q 

M 

Q 

Nov.  16,  1912- 

Plus.... 

9 

12.7 

114 

0 

90 

551 

900 

798.4 

213.38 

13.49 

July  31,  1913 

Minus  .  . 

0 

11.0 

00 

0 

50 

755 

000 

550.5 

133.47 

0.05 

'  -55' 

'i0!54 

3.50 

Aug.    1,  1913- 

Plus.... 

11 

13.2 

145 

2 

88 

364 

900 

670.3 

306.29 

17.16 

July  31,  1914 

Minus  .  . 

11 

11.5 

153 

101 

750 

000 

805.4 

219.47 

13.35 

-135' 

'51:74 

6.31 

Aug.     1,  1914- 

Plus  .... 

8 

12.9 

103 

0 

62 

329 

859 

676.1 

299.79 

19.92 

July  31,  1915 

Minus  .  . 

B 

14-1 

113 

0 

105 

047 

000 

575.0 

109.76 

0.00 

-506' 

'5ilii 

'9.46 

Aug.     1,  1915- 

Plus 

18.1 

145 

Q 

57 

233 

804 

537.8 

.soo  so 

16.85 

July  31,  1916 

Aftnua  .  . 

B 

150 

07 

504 

000 

807.7 

212.56 

-570  ' 

51.  34 

12.65 

Aug.     1,  1916- 

Plus 

60 

g 

o 

o 

40 

180 

93.3 

50.28 

13.85 

May    1,  1917 

Minus  .  . 

1 

50!  0 

50 

0 

51 

350 

000 

813.5 

184.86 

-750 

28.10 

25.62 

the  mean  reaction-times  differed  by  —102.0  ±12. 87  seconds  (table 
45).  This  is  a  difference  7.93  times  the  probable  error.  None  of  the 
8  minus  same-day  broods  had  a  reaction-time  lower  than  that  for 
the  corresponding  plus  brood.  The  mean  reaction-time  for  these 


A   PHYSIOLOGICAL   CHARACTER.  101 

minus  broods  was  200.0  ±21. 11  seconds  greater  than  that  for  the 
plus  broods  (table  46).  This  difference  was  9.46  times  the  large 
probable  error.  There  were  rather  striking  fluctuations  in  the  mean 
reaction-times  for  both  strains  (and  especially  the  minus)  during 
this  year-period.1 

While  for  10  months  of  this  year  the  minus  strain,  judged  by 
the  reproductive  index,  was  slightly,  though  very  slightly,  the  more 


'V.'1! 


l-l         4-5        8-9         12-1         4-5        8-9       12-1         4-5        8-9        12-1        4-5        8-9        \2r\ 
1913 1914  1915  1916 


1 1  <C'i!j  i'l\\ii 


2.00 

L 


1.50 
1.00 
.50 


1913  1914  1915  1916 

1 1- 1   4-5   8-9   12-1    4-5   8-9   12-1    4-5    8-9   12-1    4-5   8-9   12-1    4   ,  OQ 


FIGURE  17. — Line  757.    Reproductive  vigor. 

A.  Average  number  in  first  brood  for  the  two  strains. 

B.  Average  age  of  mother  at  time  first  brood  was  produced. 

C.  Reproductive  indices,  actual  values. 

vigorous  of  the  two  strains,  it  was  the  more  reactive  of  the  two 
during  the  only  two-month  period  of  this  year  in  which  it  was  the 
less  vigorous  of  the  two  strains  (see  figures  ISA  and  18s).  The  mean 
reaction-time  for  the  minus  strain  was  only  20  seconds  greater  than 
that  for  the  plus  strain  during  the  last  two-month  period  of  this  year. 

1  These  fluctuations  in  the  means  for  the  different  two-month  periods,  so  far  as  coincident  in 
the  two  strains,  are  due  to  the  same  environmental  influences;  and  so  far  as  they  are  not  co- 
incident they  are  presumably  due  to  the  somewhat  differential  handling  of  the  two  strains  (see 
pages  140-142).  The  material  was  handled  with  commendable  care  (mostly  by  an  assistant) 
during  this  year-period. 


102 


SELECTION    IN    CLADOCERA    ON   THE    BASIS   OF 


During  the  year  August  1915- July  1916  (36  generations  and 
505  individuals,  and  39  generations  and  486  individuals,  respectively) 
the  reaction-time  means  for  the  two  strains  showed  considerably 
greater  divergence  (table  45).  The  difference  was  —3 12.0  ±11.91 
seconds,  26.2  times  the  statistical  probable  error.  The  8  same-day 
broods  for  the  same  period  (table  46)  gave  a  mean  difference  of 
—  270  ±21. 34  seconds,  12.65  times  the  probable  error.  With  one 
exception,  each  of  the  8  same-day  broods  for  the  minus  strain  had  a 


.75 

.50 

.25 

n/\ 

< 

±L 

n 
i 

T? 

< 

i 

II        A      Ttllr 

1 

i 

.25 

.50 
onn 

- 

i 

i   -    "   i   o                           i 

II  r 

750 


600 


450 


300 


ISO 


00 


A 


A 


ll-l 
912 


4-5   8-9 
1913 


12-1 


4-5   8-9 
1914 


12-1 


4-5   8-9 
1915. 


12-1 


4-5   8-9 
I9IQ 


12-1 


4 
1917 


FIGURE  18. — Line  757. 

A.  Reproductive  indices,  superiority. 

B.  Reaction-time  curves  with  composite  curves  for  all  the  other   Simocephalus   plus  and 
minus  strains  (from  December  1914)  superimposed. 

higher  reaction-time  by  from  84  to  546  seconds.  The  test  series 
conducted  during  April  1916  consisted  of  652  individuals  of  the  plus 
strain  and  677  individuals  of  the  minus  strain  (table  45).  In  every 
one  of  these  12  pairs  of  broods  the  brood  from  the  minus  strain  had 
the  higher  reaction-time.  The  mean  reaction-time  for  each  strain 
was  relatively  high,  presumably  because  of  unusual  environmental 
conditions.  The  means  were  697.0  seconds  for  the  plus  strain  and 
864.6  seconds  for  the  minus  strain.  The  difference  (-167.6  ±9.12 
seconds)  was  18.38  times  the  probable  error,  a  difference  unmistakably 
significant.  It  can  scarcely  be  doubted  that  if  this  test  series  had 


A   PHYSIOLOGICAL   CHARACTER. 


103 


been  conducted  during  any  of  the  following  months  (under  usual 
environmental  conditions),  when  the  plus  strain  was  more  reactive, 
the  plus  strain  would  have  had  a  much  lower  mean  and  the  minus 
strain  possibly  a  somewhat,  but  probably  not  much,  lower  mean,  so 
that  the  difference  would  have  been  much  larger.1  It  is  interesting, 
however,  to  have  this  test  series  come  at  a  time  of  relatively  high 


900  r- 


750 


600 


450 


300 


ISO 


00 


5-IO 


5-10 


1913 


1914 


1915 


1916 


1917 


FIGURE  19. — Line  757. 

Reaction-time  curves  by  six-month  periods  with  similar  curves  for  Line  740  superimposed  . 

As  noted  on  pages  87  and  88,  the  minus  strain  of  Line  740  was  exceptionally  slightly  reactive 
compared  with  the  other  Simocephalus  minus  strains  (Line  757  excepted)  during  the  period 
from  April  to  September  1916,  while  the  740  plus  strain  was  exceptionally  reactive  during  Octo- 
ber 1916  to  January  1917.  Hence  the  point  representing  the  mean|for  the  minus  strain  for  the 
"5-10"  1916  period  is  exceptionally  high,  while  the  point  for  thefplusjoaean  for^the  "11-4" 
period  following  is  exceptionally  low. 

reaction-time  for  the  plus  strain  and  yet  find  the  difference  in  means 
so  markedly  significant. 

In  the  final  nine-month  period  (August  1916-April  1917)  of  the 
experiment  with  Line  757,  the  mean  for  the  minus  strain  was  double 

1  The  mean  for  the  minus  strain  in  this  test  series  was  only  5  seconds  higher  than  during 
the  earlier  test  series  conducted  2f  years  earlier  (September  1913).  On  the  other  hand,  the 
mean  for  the  plus  strain  was  288  seconds  higher  in  the  later  than  in  the  earlier  test  series.  Yet, 
in  general,  the  plus  strain  during  the  latter  part  of  the  experiment  was  very  much  more  reactive 
than  during  the  earlier  portion  of  the  experiment.  The  explanation  of  these  peculiar  means 
compared  with  the  means  for  the  selection  data  lies  in  the  following  facts:  (1)  The  plus  strain 
of  Line  757  was  more  reactive  than  normally  during  the  first  test  series.  This  was,  in  part  at 
least,  due  to  local  experimental  conditions.  Of  the  15  broods  tested  in  making  selections  (in  the 
laboratory  strains  of  the  different  species)  during  the  period  when  this  test  series  was  conducted, 
the  broods  in  11  strains  had  lower  reaction-time  means  than  the  combined  means  for  the  immedi- 
ately preceding  and  next  succeeding  broods  of  the  same  strains  and  the  differences  were  much 
larger  than  in  the  4  cases  in  which  the  reverse  relation  held.  Hence  it  is  clear  that  the  first  test 
series  for  Line  757  was  conducted  at  a  time  when  local  environmental  conditions  induced  ab- 
normally low  reaction-time  averages  in  most  of  the  Cladocera  strains.  (2)  During  the  second 
test  series  the  plus  strain  of  Line  757  was  relatively  slightly  reactive.  The  test-series  mean  for 
the  plus  strain  was  697  seconds,  a  larger  mean  than  had  occurred  in  the  selection  data  for  14 
months  and  larger  than  occurred  at  any  later  two-month  period  of  the  experiment.  Two  broods 
used  in  selection  tests  in  757  plus  soon  after  this  test  series  was  conducted  had  similar  averages, 
but  of  the  other  27  broods  of  this  strain  tested  during  the  remainder  of  the  selection  experiment 
only  one  had  an  average  as  high  as  that  for  this  entire  test  series.  It  seems  clear  that  the  757 
plus  strain  was  in  a  relatively  slightly  reactive  condition  at  the  time  of  the  second  test  series. 


104  SELECTION    IN    CLADOCERA    ON    THE    BASIS    OF 

that  for  the  plus  strain.  The  means  for  both  strains  were  lower  than 
for  any  other  longer  period  of  the  experiment  (313.1  seconds  for  the 
plus  and  625.9  seconds  for  the  minus  strain,  table  45).  The  difference 
was  -312.8 ±18.37  seconds.  The  mean  difference  in  reaction-time 
was  slightly  increased.  Because  of  the  much  smaller  total  number  of 
individuals,  however,  the  probable  error  of  the  difference  was  larger, 
though  the  difference  was  still  17.01  times  its  probable  error.  There 
was  only  one  same-day  brood.  Although  from  a  single  pair  of  broods 
little  may  wisely  be  assumed,  it  is  interesting  to  note  that  the  mean 
for  the  plus  brood  was  93  seconds,  while  that  for  the  minus  was 
814  seconds,  the  latter  mean  being  almost  9  times  the  former. 

While  the  reaction-time  curves  by  two-month  periods  (figure 
18s)  clearly  show  the  wide  and  increasing  divergence  in  reactiveness 
between  the  two  strains  of  Line  757,  curves  by  six-month  periods 
(November- April  and  May-October)  are  also  given,  together  with 
similar  curves  for  Line  740  for  comparison  with  them  (figure  19). 
Naturally  these  curves  are  less  affected  by  local  fluctuations  and  are 
smoother  curves. 

OTHER  FEATURES  OF  THE  DATA. 

Examination  of  other  features  of  the  data  for  Line  757  brings 
out  similar  evidence  of  a  marked  effect  of  selection.1  The  average 
minimum  reaction-time2  for  the  two  strains  is  a  point  on  which  their 
reactiveness  may  be  compared.  The  average  minimum  reaction- 
time  for  the  plus  strain  for  the  longer  periods  of  the  experiment  was 
progressively  lowered  from  468  to  357,  282,  190,  and  94  seconds 
(table  45).  For  the  minus  strain  there  was  likewise  a  lowering, 
though  not  so  great  nor  so  consistent.  The  minus  strain  average 
minima  were  639,  548,  380,  467,  and  245  seconds  (table  45).  For 
the  plus  strain  the  average  minimum  for  the  last  long  period  was 
only  one-fifth  that  for  the  first  of  the  5  longer  periods;  for  the  minus 
strain  the  minimum  for  the  last  period  was  two-fifths  that  for  the 
earliest  period.  The  average  minima  for  the  two  strains  show  wide 
differences  throughout  the  experiment,  the  differences  being  171, 
191,  98,  277,  and  151  seconds  for  the  5  longer  periods.  These  differ- 

1In  a  single  minor  detail  the  data  for  a  small  portion  of  the  time  do  not  seem  consistent 
with  an  effect  of  selection.  This  point  is  the  relative  numbers  of  negatively  reacting  individuals 
in  the  two  strains.  From  August  to  December  1915  there  were  considerably  more  of  the  nega- 
tively reacting  individuals  in  the  plus  (21)  than  in  the  minus  (5)  strain.  This  is  of  course  con- 
trary to  one's  expectation  in  a  less-reactive  strain.  However,  19  of  the  21  negatively  reacting 
individuals  in  the  plus  strain  for  this  period  occurred  in  four  broods.  Since  in  all  strains  negatively 
reacting  individuals  are  extremely  irregular  in  their  occurrence,  and  when  they  do  occur  are  often 
relatively  numerous  within  a  single  brood,  it  seems  probable  that  these  reactions  are  in  large 
measure  determined  by  environmental  conditions  and  hence  are  of  little,  if  any,  real  significance 
(cf.  page  15).  If,  however,  they  were  to  be  ascribed  an  important  significance  here,  this  data  alone 
(and  only  a  very  limited  portion  of  it)  runs  counter  to  the  plain  implication  of  all  the  rest  of 
the  data  for  Line  757.  During  the  remainder  of  the  experiment  the  plus  and  minus  strains  of 
Line  757  had  approximately  equal  numbers  of  negatively  reacting  individuals  (tables  43  and  44). 

2  This,  of  course,  includes  the  reaction-time  of  only  a  single  individual  (the  most  reactive 
one)  of  each  brood.  Like  the  data  for  the  average  maximum  reaction-time,  it  has  much  less 
value  as  a  measure  of  reactiveness  than  the  mean  reaction-time  for  all  the  individuals  tested; 
nevertheless,  it  is  highly  significant. 


A   PHYSIOLOGICAL   CHARACTER.  105 

ences  are  more  instructive  if  stated  in  percentages.  The  average 
minima  are  37,  54,  35,  146,  and  161  per  cent  higher  for  the  minus 
than  for  the  plus  strain. 

These  differences  clearly  point  to  a  marked  and  cumulative  di- 
vergence in  reactiveness  of  the  most  reactive  individuals  of  the  two 
strains,  a  divergence  which  became  in  the  final  period  more  than 
3  times  as  great  as  the  average  minimum  for  the  plus  strain.  It  is 
a  striking  fact  that  for  the  last  9  months  of  the  experiment  the  mean 
reaction-time  for  the  plus  strain  was  considerably  lower  than  the 
average  minimum  reaction-time  for  the  broods  of  the  same  strain  for 
the  first  21  months  of  the  experiment. 

A  comparison  of  the  average  maximum  reaction-times  is  less 
instructive,  for  a  large  percentage  of  the  broods  of  S.  exspinosus 
contain  over-time  individuals;  but  even  here  comparison  indicates 
increasingly  greater  reactiveness  for  the  least  reactive  individuals  of 
the  broods  of  the  plus  strain  toward  the  end  of  the  experiment.  For 
the  first  9  months  every  brood  in  the  plus  strain  as  well  as  the  minus 
strain  contained  over-time  individuals  and  the  average  maximum 
reaction-time  for  each  strain  is  (the  arbitrary)  900  seconds.  For  the 
longer  periods  of  the  entire  experiment  the  maxima  for  the  plus  strain 
declined  from  900  to  879,  829,  811,  and  632  seconds  (table  45). 
There  is  a  slight  decline  for  the  minus  strain,  the  averages  being  900, 
884,  854,  888,  and  854  seconds  (table  45).  The  difference  in  average 
maximum  reaction-time  for  the  two  strains  increased  from  0  to  5, 
25,  77,  and  222  seconds.  The  average  maximum  reaction-times  are 
0,  1,  3,  10,  and  35  per  cent  higher  for  the  minus  than  for  the  plus 
strain.  For  the  minus  strain  the  total  lowering  of  the  average  maxi- 
mum reaction-time  was  only  46  seconds,  or  5.1  per  cent  as  compared 
with  268  seconds,  or  29.8  per  cent,  for  the  plus  strain. 

The  data  for  the  same-day  broods  may  be  referred  to  again. 
They,  too,  show  an  interesting  decline  in  the  mean  for  the  plus 
strain  through  the  various  periods  (from  798  to  670,  676,  538,  and  93 
seconds,  table  46).  The  average  for  the  last  period  is  for  a  single 
brood  and  is  abnormally  low.  The  general  trend  is  unmistakable, 
however.  There  is  no  marked  lowering  for  the  minus  strain,  the 
means  being  857,  805,  876,  808,  and  814  seconds  (table  46).  The  in- 
creased divergence  between  the  means  for  the  two  strains  is  striking— 
from  58  to  135,  200,  270,  and  720  seconds.  Stated  in  percentages, 
these  differences  for  the  several  longer  periods  of  the  experiment, 
compared  with  the  mean  for  the  plus  strain,  are  7.3,  20.2,  29.6,  50.1, 
and  774.2  per  cent.  Again,  although  allowing  that  the  difference  for 
the  last  period  was  obtained  from  too  meager  data,  the  divergence 
is  marked  and  continuous,  and  these  differences  arise  from  the  most 
strictly  comparable  data  obtainable. 

Considering  the  number  of  " over-time"  individuals  for  the  two 
strains  in  the  different  periods  of  the  selection  experiment  is  another 


106  SELECTION   IN   CLADOCERA   ON   THE   BASIS   OF 

method  of  checking  up  the  data  (table  47).  This  is  a  test  of  vital 
significance.  The  number  of  over-time  individuals  is  extremely 
significant  of  the  reactiveness  of  a  strain.  For  the  5  longer  periods 
of  the  experiment  the  figures  for  the  plus  strain  are  247,  302,  216, 
143,  and  37  over-time  individuals;  for  the  minus  strain  227,  320,  334, 
376,  and  151  individuals.  These  can  be  compared  better  on  a  per- 
centage basis.  The  percentages  of  over-time  individuals  for  the 
plus  strain  for  the  5  longer  periods  of  the  experiment  are  77.2,  59.9, 
45.8,  28.3,  and  15.1  per  cent;  for  the  minus  strain  87,  77.7,  65.9, 
77.4,  and  57.2  per  cent.  Starting  near  equality  (though  the  plus 
strain  almost  from  the  start  had  significantly  fewer  over-time  indi- 
viduals), the  differences  are  9.8,  17.8,  20.1,  49.1,  and  42.1  per  cent. 

Hence  the  percentage  of  over-time  individuals  in  the  plus  strain 
decreased  throughout  the  experiment  and  particularly  during  the 
last  21  months  of  the  experiment.  The  final  percentage  was  less  than 
one-fifth  that  for  the  earliest  longer  period.  For  the  minus  strain 
there  was  also  a  decrease,  but  it  was  less  consistent,  and  in  the  end 
the  percentage  of  over-time  individuals  was  two-thirds  as  large  as 
at  the  beginning  (see  table  47). 

It  is  possibly  worth  while  to  consider  the  mean-reaction  times 
of  the  more  reactive  individuals  of  the  two  strains  of  Line  757, 
omitting  from  consideration  for  a  moment  the  over-time  individuals. 
While  this  is  a  point  of  interest,  the  fact  remains  that  the  relative 
number  of  over-time  individuals  is  in  itself  as  good  a  measure  of 
reactiveness  as  might  be  desired.  The  large  influence  of  the  arbitrary 
reaction -times  of  900  seconds,  assumed  for  the  over-time  indi- 
viduals, does  not  ascribe  too  much  influence  in  determining  the  mean 
reaction-time.  In  point  of  fact,  the  reverse  is  true.  This  arbitrary 
method  of  assuming  900  seconds  as  the  reaction-time  for  the  over- 
time individuals  minimizes  the  rightful  influence  upon  mean  re- 
action-time of  these  relatively  non-reactive  individuals.  The  data 
are  given  in  table  47  and  show  that  there  is  a  progressive  reduction 
in  the  mean  reaction-time  for  the  more  reactive  individuals  of  both 
strains,  considered  by  the  longer  periods  of  the  experiment.  This 
reduction  for  the  minus  strain  is  from  431  to  376,  334,  326,  and  260 
seconds;  but  for  the  plus  strain  the  reduction  is  larger,  from  410  to 
360,  356,  284,  and  209  seconds.  The  differences  between  the  means 
for  the  two  strains,  with  some  irregularity,  became  larger,  the  plus 
strain  having  the  lower  reaction-time.  These  differences  were  as 
follows:  -21,  -16,  +22,  -43,  and  -51  seconds.  The  reduction 
in  mean  reaction-time  for  the  plus  strain  in  the  final  period  as  com- 
pared with  the  first  period  of  the  experiment  was  201  seconds;  for 
the  minus  strain,  171  seconds.  The  differences  for  the  last  two 
longer  periods  of  the  experiment  have  statistical  significance,  being 
respectively  2.97  and  4.2  times  their  probable  errors. 


A   PHYSIOLOGICAL   CHARACTER. 


107 


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:  I   :  I  +  :  :  i 


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d  QO  oJ  >-<'  d  cs  eo  ^'  <N  ci 


3 


0<  05  N  05  t>.  05 


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-notional  jo  eajimbs  jo  rang 


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^  ^  TO  »1  iH  >^ 


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CO  ~*(N  90  "1 


:  3  :  3  :  3  :  3  :  3 


108  SELECTION   IN    CLADOCERA    ON   THE   BASIS   OF 

Thus,  for  the  moment  ignoring  the  data  for  the  less  reactive 
(over-time)  individuals,  one  finds  that  the  more  reactive  individuals 
of  the  plus  strain  showed  greater  reactiveness  than  the  more  reactive 
individuals  of  the  minus  strain.  These  differences  are  not  large  and 
are  not  all  in  the  same  direction,  but  on  the  basis  of  this  portion  of 
the  data  alone  there  is  evidence  for  an  effect  of  selection,  though 
naturally  it  is  not  as  decisive  as  that  obtained  from  the  complete 
data. 

RELATION  BETWEEN  RELATIVE  VIGOR  AND  MEAN  REACTION-TIME. 

There  remains  to  be  considered  the  relation,  if  any,  between  the 
relative  vigor  of  the  two  strains  and  their  relative  reactiveness. 
This  is  an  important  point.  While  it  has  already  been  shown  that 
with  the  other  lines  subjected  to  selection  in  connection  with  which 
this  point  was  considered,  there  is  little,  if  any,  relation  between 
reactiveness  and  the  general  vigor  of  the  stock,  it  still  remains 
desirable  to  know  whether  such  is  certainly  the  case  with  Line  757. 

There  is  to  be  noted  with  the  minus  strain  of  Line  757,  as  with 
the  minus  strains  of  many  other  lines  of  Cladocera  subjected  to 
selection,  a  somewhat  lower  reproductive  vigor  than  with  the 
corresponding  plus  strain.  This  is  presumably  the  result  of  the 
individuals  selected  in  the  minus  strains  being  somewhat  below  the 
average  in  vigor.  That  this  at  the  same  time  has  not  operated 
materially  to  increase  the  mean  reaction-time  of  the  minus  strain  is 
evident  enough  in  nearly  every  line.  The  data  for  vigor  and  mean 
reaction-time  considered  for  the  other  individual  lines  (and  for  all 
the  lines  as  a  whole)  seem  to  show  conclusively  that  there  is  no 
relation  between  vigor  and  reaction-time.  While  this  suggests  that 
in  Line  757  the  result  of  selection  is  not  due  to  less  vigor  in  the  minus 
strain,  yet  it  does  not  preclude  the  possibility  that  it  may  have  been 
a  factor  in  the  divergence  in  reactiveness  in  Line  757.  Some  attention 
will  now  be  directed  toward  an  analysis  of  the  data  on  mean  reaction- 
time  as  related  to  vigor  in  this  line.  The  matter  of  concern  is  whether 
there  is  a  relation  between  the  relative  vigor  of  the  two  strains  and 
their  mean  reaction-times,  and,  if  there  is  a  relationship,  to  what 
extent  it  is  operative.  Several  more  or  less  disconnected  considera- 
tions will  be  taken  up  in  regard  to  this  point  before  referring  to 
the  statistical  correlations. 

The  two  strains  were  compared  as  to  general  vigor  as  revealed 
by  the  measures  applied  to  them  (see  page  42)  and  stated  in  terms 
of  the  reproductive  index  (tables  43  and  44). 

1 .  The  plus  strain  was  on  the  whole  the  more  vigorous  of  the  two, 
as  will  be  seen  by  reference  to  tables  43  and  44  and  figure  ISA.  Of 
the  27  periods  of  2  months  each  (except  the  first  period  of  3  months 
and  the  last  period  of  1  month)  the  plus  strain  was  of  superior  vigor 
during  16  periods  and  the  minus  strain  during  11  periods.  The 


A   PHYSIOLOGICAL   CHARACTER.  109 

quantitative  differences  in  favor  of  the  plus  strain  averaged  greater 
than  those  for  the  minus  strain,  so  that  on  the  whole  the  plus  strain 
averaged  somewhat  higher  in  reproductive  vigor.  For  the  first  11 
periods  (23  months)  this  superiority  in  vigor  in  the  plus  strain  was 
marked;  but,  during  the  remainder  of  the  experiment  (16  two-month 
periods)  during  which  the  divergence  in  mean  reaction-time  became 
very  large,  the  average  superiority  of  the  plus  strain  in  reproductive 
vigor  was  slight  indeed,  and,  in  fact,  during  9  of  these  16  periods  the 
minus  strain  was  the  more  vigorous.  This  at  once  throws  serious 
doubt  upon  the  possibility  of  explaining  any  portion  of  the  diver- 
gence in  mean  reaction-time  as  a  result  of  a  difference  in  vigor. 

2.  Near  the  middle  of  the  curve  for  5  successive  two-month 
periods  the  minus  strain  showed  (see  figure  18,  A  and  B)  a  slight 
superiority  in  reproductive  vigor  over  the  plus  strain,  and  during 
this  time  the  minus  curve  twice  approached  the  plus  curve,  although 
in  spite  of  its  greater  vigor  the  minus  strain  had  a  consistently 
higher  reaction-time.  Examination  of  the  curves  at  this  point  might 
at  first  sight  seem  to  suggest,  however,  that  there  was  actually  some 
effect  upon  mean  reaction- time  of  the  relatively  greater  vigor  of 
the  minus  strain.  At  two  points  the  curves  covering  this  limited  period 
are  separated  by  spaces  representing  differences  in  reaction-times  of 
only  64  and  20  seconds,  but  at  the  other  three  points  the  curves  are 
separated  by  219,  133,  and  101  seconds  (table  44).  These  last  three 
divergences  are  larger  than  had  occurred  except  in  3  of  the  1 1  preced- 
ing periods  of  the  experiment.  The  mean  reaction-time  for  all  the 
data  for  the  plus  strain  during  this  ten-month  period  of  superior 
vigor  on  the  part  of  the  minus  strain  was  613.7  seconds,  119.2  seconds 
(19  per  cent)  less  than  that  for  the  minus  strain,  whose  reproductive 
index  during  this  time  averaged  0.10  higher  than  that  for  the  plus 
strain.  This  may  be  compared  (a)  with  similar  averages  for  all 
data  for  Line  757  obtained  previous  to  October  1914,  during  which 
the  mean  reaction-times  differed  by  68.7  seconds  (10  per  cent),  the 
minus  strain  having  the  higher  reaction-time  and  a  less  vigor  by 
0.25;  (b)  with  the  period  of  10  months  immediately  following  the 
ten-month  period  of  superior  reproductive  ability  on  the  part  of  the 
minus  strain,  in  which  the  plus  strain  was  the  superior  in  vigor  by 
0.19  and  its  reaction-time  averaged  325  seconds  (71  per  cent)  less; 
and  (c)  with  the  final  6  periods  (11  months),  during  which  the  plus 
strain  had  a  lower  reproductive  index  by  only  0.08  and  a  lower  mean 
reaction-time  by  300.6  seconds  (nearly  89  per  cent).  This  shows 
that  the  rapidly  increasing  percentage  difference  between  the  mean 
reaction- times  was  quite  independent  of  the  relative  vigor  of  the  two 
strains. 

Further,  in  the  2  two-month  periods  immediately  preceding  the 
5  periods  of  superior  vigor  on  the  part  of  the  minus  strain  just 
referred  to,  the  minus  strain  showed  a  greater  reactiveness  to  light 


110       SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

than  the  plus  strain  in  one  case  and  a  practically  equal  reactiveness 
in  the  other  case — a  thing  which  did  not  occur  elsewhere  in  the 
experiment — and  yet  the  plus  strain  was  markedly  superior  in  vigor. 

3.  There  were  10  two-month  periods  during  which  the  vigor  of 
the  plus  strain  was  quite  above  the  average  (figure  17c)  the  repro- 
ductive index  being  more  than  1.50.     The  mean  reaction-time  for 
these  periods  of  high  vigor  was  546.8  seconds,  36  seconds  less  than 
the  mean  for  the  plus  strain  for  the  entire  period  of  the  experiment. 
There  were  9  corresponding  periods  of  high  vigor  for  the  minus 
strain,  for  which  the  mean  reaction-time  was  768  seconds,  23  seconds 
greater  than  the  mean  for  the  entire  period  for  this  strain.     There 
were  9  two-month  periods  during  which  the  plus  strain  had  a  repro- 
ductive index  under  1.25,  somewhat  below  the  average.     For  these 
periods  the  general  average  reaction-time  was  634.9  seconds,  52 
seconds  above  the  average  for  the  entire  experiment.     For  12  periods 
of  correspondingly  low  reproductive  indices  in  the  minus  strain  the 
mean  reaction-time  was  770.8  seconds,  26  seconds  above  the  average 
for  the  whole  experiment.     Hence  in  3  of  these  4  comparisons  the 
mean  reaction-times  varied  in  the  direction  to  indicate  some  relation 
between  actual  vigor  and  mean  reaction-time.     In  one  case  the 
reverse  is  true. 

4.  A  fairer  method  of  making  a  comparison  between  relative 
vigor  and  mean  reaction-times  is  a  detailed  period-by-period  exami- 
nation of  the  data  for  reproductive  index  and  the  mean  reaction- 
time.     Comparing  the  mean  reaction-time  for  the  plus  strain  for  the 
10  periods  of  high  reproductive  indices  (above  1.50)  with  the  reaction- 
time  means  for  the  adjacent  points  in  the  curve  (each  representing 
two-month  periods),  it  is  seen  (figures  17c  and  18fi)  that  in  2  cases 
the  reaction-time  means  are  higher  than  in  the  adjacent  points  in 
the  curve,  in  3  cases  intermediate,  and  in  5  cases  lower.     For  the  9 
periods  of  like  high  reproductive  indices  for  the  minus  strain,  the 
mean  reaction- times  are  higher  than  in  the  adjacent  portions  of  the 
curves  in  5  cases,  intermediate  in  1  case,  and  lower  in  3  cases.     A 
similar  comparison  for  9  periods  of  low  reproductive  index  (below 
1.25)  in  the  plus  strain  shows  that  in  5  cases  the  mean  reaction-time 
is  higher  than  in  adjacent  points  in  the  curve,  and  in  4  cases  it  is 
lower.     For  the  12  periods  of  like  low  reproductive  indices  in  the 
minus  strain  the  mean  reaction-times  are  higher  than  in  adjacent 
portions  of  the  curve  in  3  cases,  intermediate  in  5  cases,  and  lower  in 
4  cases.     These  comparisons  indicate  a  possible  slight  relation  be- 
tween mean  reaction-time  and  reproductive  vigor  for  the  plus  strain, 
high  vigor  being  more  often  associated  with  greater  reactiveness  and 
a  lower  reaction-time;  but  the  reverse  relation  is  more  strongly 
indicated  for  the  minus  strain,  higher  reproductive  vigor  being  more 
often  associated  with  less  reactiveness  and  a  higher  reaction-time. 


A   PHYSIOLOGICAL   CHARACTER.  Ill 

5.  Arbitrarily  selecting  the  6  periods  of  highest  reproductive 
indices  for  the  plus  strain,  they  have,  as  compared  with  adjacent 
parts  of  the  curve,  a  higher  reaction-time  twice,  an  intermediate 
reaction-time  once,  and  a  lower  reaction-time  3  times.     The  minus 
strain  for  its  6  highest  reproductive  indices  has  a  higher  reaction- 
time  4  times,  an  intermediate  reaction-time  once,  and  a  lower  reac- 
tion-time once.     The  5  periods  of  lowest  reproductive  indices  for 
the  plus  strain  have,  as  compared  with  adjacent  points  in  the  curve, 
a  higher  reaction-time  3  times  and  a  lower  reaction-time  twice. 
The  similar  periods  for  the  minus  strain  have  a  higher  reaction-time 
in  one  case,  an  intermediate  reaction-time  in  3  cases,  and  a  lower 
reaction-time  once. 

The  comparisons  under  (4)  and  (5)  show  some  evidence  of  a 
relation  between  vigor  and  reaction-time  in  the  plus  strain,  the  mean 
reaction-time  more  often  varying  in  the  direction  anticipated,  if 
greater  vigor  is  associated  with  greater  promptness  in  reaction  (and 
hence  lower  reaction-time)  than  in  the  reverse  direction.  But  with 
the  minus  strain  the  differences  are  such  as  to  indicate  the  reverse 
relation,  i.  e.,  greater  vigor  associated  with  less  promptness  in  re- 
action (higher  reaction-time).  These  small  differences  about  neu- 
tralize each  other  and  lead  one  to  conclude,  from  this  portion  of 
the  evidence,  that  there  is  probably  no  relation  between  vigor  and 
reaction- time.1 

6.  For  the  two-month  period  during  which  the  minus  strain 
showed  its  greatest  vigor  (October— November  1914)  its  mean  reaction- 
time  was  greater  than  at  any  other  period  except  one,  and  much 
greater  than  the  average  for  the  succeeding  5  periods,  during  which 
its  reproductive  index  was  about  35  per  cent  lower. 

7.  It  is  worthy  of  note  that  4  of  the  6  high  points  in  the  curve 
of  the  reaction-times  of  the  minus  strain  occur  when  the  minus 
strain  was  superior  in  vigor  to  the  plus  strain.     This,  if  considered 
alone,  would  seem  to  suggest  a  reverse  relation  between  reproductive 
vigor  and  reaction-time,  a  relation  which  would  seem  not  to  have 
biological  significance. 

8.  Attention  may  be  directed  to  the  two-month  periods  during 
which  (even  after  the  effect  of  selection  appeared  already  to  have 
become  established)  the  minus  strain  had  a  reaction-time  approxi- 
mating or  lower  than  that  for  the  plus  strain.     For  the  period  June- 
July  1914,  during  which  the  minus  strain  was  on  the  average  the 
more  reactive  of  the  two,  and  the  following  two-month  period  during 
which  the  minus  strain  was  the  less  reactive  by  only  2  seconds,  the 
plus  strain  was  markedly  the  more  vigorous.     For  the  February- 
March  1915  period,  when  the  minus  strain  was  within  64  seconds 
of  the  plus  strain,  the  minus  strain  was  slightly  the  more  vigorous. 

1  These  data,  however,  might  be  taken  to  offer  slight  corroboration  of  the  puzzling  results 
obtained  from  the  correlations  based  upon  this  data  (see  page  113). 


112  SELECTION   IN   CLADOCERA   ON   THE   BASIS   OF 

For  the  following  June-July  period,  during  which  the  minus  strain 
was  within  20  seconds  of  being  as  reactive  as  the  plus  strain,  it  again 
was  slightly  the  more  vigorous.  But  it  is  noteworthy  that  in  the 
period  intervening  between  the  two  periods  just  referred  to  the 
minus  strain  was  superior  in  vigor  to  the  plus  strain  by  a  difference 
3  times  as  great  as  in  the  other  two  periods,  and  yet  the  reaction-time 
for  the  minus  strain  was  101  seconds  greater  than  that  for  the  plus 
strain.  For  the  last  two-month  period  (December  1916-January 
1917),  in  which  the  reaction-time  mean  for  the  minus  approached 
that  for  the  plus  strain,  the  minus  was  considerably  the  more  vigorous. 
It  is  noteworthy  here,  however,  that  in  the  period  just  preceding1 
the  minus  strain  showed  greater  superiority  in  vigor  than  at  any 
other  time,  and  yet  its  reaction-time  differed  from  that  for  the  plus 
by  a  larger  margin  (629  seconds)  than  for  any  other  period  of  the 
experiment.  This  analysis  might  be  prolonged  further,  but  enough 
has  been  seen  to  make  clear  that  there  is  at  most  a  very  slight  rela- 
tion between  the  reproductive  index  and  the  mean  reaction-time. 

Of  the  points  discussed  above,  (3)  and  doubtfully  (4)  and  (5) 
indicate  a  possible  effect  of  vigor  upon  reaction-time  in  the  plus 
strain  (but  not  in  the  minus  strain) — i.  e.,  the  greater  the  vigor  the 
more  reactive  the  individuals  and  the  lower  the  reaction-time.  But 
this  bit  of  evidence  is  not  very  convincing,  since  the  numerical  differ- 
ences in  the  comparisons  made  are  small  and  the  differences  in 
averages  are  not  all  in  the  right  direction  to  bear  out  such  a  relation- 
ship. On  the  other  hand,  (1),  (2),  and  (8)  fail  to  show  the  appearance 
of  an  effect  of  mean  vigor  upon  mean  reaction-time,  while  (5)  (in 
part),  (6),  and  (7)  seem  to  indicate  the  reverse  effect,  that  greater 
vigor  is  associated  with  lessened  reactiveness  and  a  higher  reaction- 
time. 

Obviously,  the  best  method  of  determining  the  relation,  if  any, 
between  reproductive  vigor  and  mean  reaction-time  is  by  statistical 
correlation.  The  mean  reaction-times  and  mean  reproductive 
indices,  both  by  two-month  averages,  were  correlated  for  the  data  for 
the  Line  757  strains,  using  as  the  terms  of  the  correlation  the  repro- 
ductive index  of  the  mother  and  the  mean  reaction-time  of  her  first 
brood  of  offspring.  The  correlation  is  -0.1 109  ±0.127  for  the  plus 
strain  and  0.0612  ±0.129  for  the  minus  strain.  In  order  to  have  a 
larger  series,  the  data  for  both  strains  of  Line  757  were  thrown 
together.2  The  resulting  correlation  is  -0.12845  ±0.0902.  These 
correlations  are  low,  and,  judged  by  their  probable  errors,  are  not  of 
statististical  significance. 

1The  numbers  of  individuals  tested  during  this  period  were  abnormally  small,  so  that  these 
averages  are  not  as  trustworthy  as  most  of  the  others,  but  the  major  part  of  this  difference  can 
not  be  regarded  as  accidental. 

1  Since  the  two  strains  are  different  in  their  reactiveness  to  light,  this  combination  of  data  is 
open  to  criticism,  although  it  is  reasonable  to  suppose  that  variations  in  vigor  should  influence 
the  reaction-time  in  the  two  strains  similarly. 


A   PHYSIOLOGICAL   CHARACTER.  113 

Correlations  involving  the  individual  reaction-times  and  the  re- 
productive indices  of  the  mothers  were  made.  For  the  plus  strain 
this  data  included  every  individual  of  Line  757  whose  reaction-time 
was  obtained  in  making  the  selections  and  the  reproductive  index 
of  whose  mother  was  known — a  total  of  1,992  individuals.  The 
correlation  was  —0.000828 ±0.0151.  For  the  minus  strain  (with 
1,842  individuals)  the  corresponding  correlation  was  0.2154  ±0.0149. 

A  negative  correlation  between  vigor  and  reaction-time  would 
have  obvious  biological  significance,  while  a  positive  correlation  would 
not,  for  it  is  difficult  to  see  how  less  vigor  would  be  causally  asso- 
ciated with  greater  reactiveness  (lower  reaction- time) .  Both  the 
correlations  for  the  757  plus  strain  are  negative,  but  although  sug- 
gestive, they  are  much  too  small  to  be  statistically  significant.  The 
correlations  for  the  757  minus  strain  are  both  positive  and  the 
second,  while  not  large,  is  14  times  its  probable  error.  This  is  not 
convincing  evidence,  but  on  its  face  it  indicates  that  in  the  minus 
strain  of  Line  757  the  greater  the  vigor  the  higher  the  reaction- time; 
that  is  to  say,  a  greater  vigor  is  associated  with  less  reactiveness  and 
a  lower  vigor  with  greater  reactiveness.  Some  possible  corroboration 
of  this  anomalous  state  of  affairs  is  seen  under  (4)  and  (5)  above 
(page  HO),1  but  it  is  difficult  to  understand  how  this  relation  is 
possible. 

Further  analysis  of  the  data  for  Line  757  was  attempted.  Cor- 
relations were  made  between  age  of  the  mother  at  the  time  of  pro- 
ducing her  first  brood  and  the  reaction-times  of  her  young;  between 
the  number  in  the  brood  (size  of  brood)  and  their  reactiveness,  and 
between  the  age  of  the  mother  and  the  size  of  her  first  brood. 

The  correlations  between  the  age  of  mothers  and  reactiveness 
of  young  were  -0.03865  ±0.0205  for  the  plus  strain  (1,992  young) 
and  -0.01529  ± 0.0157  (1,842  young)  for  the  minus  strain.  Between 
the  number  in  the  brood  and  their  reactiveness  the  correlations 
were  -0.07619  ±0.0150  for  the  plus  strain  and  0.01606  ±0.0157  for 
the  minus  strain. 

Other  things  being  equal  (temperature,  food,  etc.),  the  more 
vigorous  mothers  reproduce  earlier  and  the  less  vigorous  mothers 
later.  Hence  a  correlation  of  obvious  biological  significance  between 
age  of  mothers  and  reactiveness  of  young  should  be  positive.  Both 
the  correlations  obtained  were  negative,  but  they  were  very  low,  and 
neither  was  of  statistical  significance.  A  biologically  significant  cor- 
relation between  size  of  brood  and  reaction-time  should  apparently 
be  negative.  The  correlation  obtained  for  the  data  for  the  plus 
strain  was  negative  and  was  5  times  its  probable  error,  but  it  is 
numerically  so  small  (—0.07619)  that  it  at  most  indicates  an  ex- 
tremely slight  relation  between  size  of  brood  and  reaction-time.  The 

1  Likewise,  this  fits  with  the  observation  recorded  under  (7)  stated  on  page  111. 


114  SELECTION   IN    CLADOCERA    ON    THE    BASIS    OF 

correlation  obtained  for  the  same  data  for  the  minus  strain  is  positive 
and  is  too  small  to  be  of  statistical  significance.1 

Considered  on  all  points,  there  is  little  in  the  data  or  the  course 
of  the  curves  to  indicate  any  appreciable  relationship  between  the 
relative  vigor  of  the  two  strains  and  their  mean  reaction-times.  The 
comparisons  made  and  the  correlations  calculated  show  that  there 
is  at  most  only  a  possible  slight  relation  between  average  vigor 
(reproductive  index)  and  mean  reaction-time.  The  relation  (if 
actually  significant)  is  so  small,  however,  that  when  the  data  are 
examined  in  many  detailed  ways  it  usually  quite  fails  to  appear  and 
statistical  correlations  do  not  establish  it.  This  possible  relation  is 
large  enough  at  most  to  account  for  only  a  small  portion  of  the  fluc- 
tuations of  the  curves  and  at  best  for  only  a  small  fraction  of  the 
divergence  between  the  mean  reaction-times  of  the  two  strains  of 
Line  757.  Hence  it  becomes  clear  that  fluctuations  of  the  mean 
reaction-time  curves  of  the  two  strains  were  controlled  neither  in 
direction  nor  in  amount  (to  any  appreciable  extent  at  any  rate) 
by  the  relative  vigor  of  the  two  strains.2 

While  the  writer  is  convinced  that  in  Line  757  there  is  at  most 
only  a  very  slight  relation  between  vigor  and  reaction-time  and  has 
already  laboriously  presented  the  evidence  on  this  point  from  the 
data  for  Line  757  itself,  this  conclusion  is  perhaps  even  better  justi- 
fied from  consideration  of  the  data  for  the  plus  and  minus  strains 
of  the  other  Cladocera  lines  subjected  to  selection.  Inspection  of  the 
curves  showing  mean  reaction-times  and  reproductive  indices  for  the 
other  lines  of  Simocephalus  exspinosus3  brings  some  crucial  evidence 
to  bear  directly  on  this  point. 

The  data  for  Line  794  (tables  33  and  34,  and  figures  11,  A,  B, 
and  c)  show  considerably  greater  reproductive  superiority  on  the  part 
of  the  plus  strain  (as  compared  with  the  corresponding  minus  strain) 
than  in  the  plus  strain  of  Line  757,  yet  the  reaction-time  means  for  the 
two  strains  of  Line  794  run  much  nearer  together  than  in  Line  757. 

The  data  for  Lines  795  (figures  12,  A,  B,  and  c)  and  796  (figures  13, 
A,  B,  and  c)  show  still  greater  differences  in  mean  reproductive  vigor  of 
their  plus  and  minus  strains,  yet  there  are  no  corresponding  differences 
in  mean  reaction-time.  The  differences  in  mean  reaction-time  which 
occur  in  different  parts  of  the  curves  for  these  lines  do  not  at  all  lend 

1  The  correlations  between  age  of  mother  and  size  of  brood  have  no  apparent  bearing  upon 
reactiveness,  but  they  may  be  mentioned  for  their  biological  interest.    The  correlations  obtained 
are  -0.05785  db  0.0525  for  the  plus  strain  and    -  0.01732  ±  0.0527  for  the  minus  strain.     If  one 
eliminates  the  data  for  mothers  which  did  not  produce  their  first  broods  until  more  than  11  days 
of  age,  thus  eliminating  the  obviously  weak  mothers  (since  more  than   11   days  is  an  abnor- 
mally late  reproductive  age),  the  correlations  are  0.11618  ±  0.0531    for    the  plus  strain  and 
0.12409  db  0.0533  for  the  minus  strain. 

2  It  is  obvious  that  if  the  vigor  of  the  stock  were  extremely  low  the  mean  reaction-times 
should  become  higher,  but  this  stock  was  always  maintained  at  the  highest  possible  vigor  and 
the  varying  reproductive  indices  merely  indicate  various  degrees  of  the  always  (except  for  very 
limited  periods)  successful  maintenance  of  high  levels  of  reproductive  vigor. 

3  See  pages  44-45,  60  and  65  and  figures  2,  B  and  c,  3,  B  and  c,  7,  B  and  c,  and  8,  B  and  c, 
for  treatment  of  this  data  for  the  D.  pulcx  lines. 


A   PHYSIOLOGICAL   CHARACTER.  115 

themselves  to  explanation  as  due  to  differences  in  relative  vigor.  For 
example,  during  the  period  from  October  1915  to  May  1916,  inclusive, 
the  plus  strains  of  Lines  795  and  796  were  both  greatly  superior  in 
reproductive  vigor  to  the  minus  strains  of  the  same  lines  (figures 
12,  A  and  B,  and  13,  A  and  B),  yet  during  this  time  the  plus  strain  of 
Line  795  was  much  less  reactive  than  its  minus  strain,  while  the  plus 
strain  of  Line  796  was  more  reactive  than  its  minus  strain. 

The  curves  for  Line  740  (figures  14s  and  15)  are  also  instructive 
on  this  point.  In  general,  the  minus  strain  averaged  slightly  less 
reactive  than  the  plus,  yet  it  was  on  the  whole  consistently  the  more 
vigorous. 

It  is  obvious  that  reaction-time  and  reproductive  vigor  are  on  the 
whole,  very  slightly,  if  at  all,  related  in  S.  exspinosus,  and  that 
differences  in  vigor  do  not  explain  the  divergence  in  mean  reaction- 
time  so  pronounced  in  Line  757. 

Further,  there  is  no  basis  for  an  assumption  that  the  reaction- 
time  differences  which  arose  in  Line  757  are  due  to  differences  in 
swimming  ability.  If  the  difference  in  reaction-time  in  Line  757 
arose  from  differences  in  swimming  ability,  the  results  are  fully  as 
interesting  from  a  genetic  standpoint  as  though  they  were  due  to 
differences  in  reactiveness  to  the  light  per  se.  But  differences  in 
swimming  ability  were  not  the  source  of  the  differences  in  reaction- 
time  between  the  two  strains  of  Line  757.  As  compared  with  the 
plus  strains,  individuals  of  the  minus  strain  less  often  showed  any 
tendency  to  react  to  light,  i.e.,  many  more  were  immobile  throughout 
the  time  of  the  test,  and  individuals  of  the  minus  strain  which  did 
reach  an  end  of  the  tank  in  general  did  so  after  more  and  longer  inter- 
ruptions of  swimming  than  occurred  for  individuals  of  the  plus 
strain.  It  is  not  believed  that  anyone  who  observed  the  behavior 
of  individuals  of  these  two  strains  in  the  experimental  tank  and  in 
the  culture-bottles  would  countenance  any  suggestion  that  the  dif- 
ferences between  the  two  strains  were  due  to  differences  either  in 
general  activity  or  in  swimming  ability. 

SPECIAL  FEATURES  OF  THE  REACTION-TIME  CURVES. 

The  approximation  of  the  curves  (figure  ISA)  for  mean  reaction- 
times  in  Line  757,  during  1914  and  1915,  at  first  sight  appears  more 
significant  than  it  really  is.  For  1  two-month  period,  June-July 
1914,  the  minus  strain  was  more  reactive  than  the  plus  strain,  and 
for  the  following  two-month  period  the  difference  was  only  —  2 
seconds,  while  for  the  June- July  1915  period  the  plus  strain  averaged 
only  20  seconds  lower  in  reaction-time  than  the  minus  strain;  but  in 
the  intervening  February-March  period,  for  which  the  curves  appear 
so  nearly  to  approach,  there  is  a  difference  of  64  seconds,  or  16  per 
cent.  That  such  fluctuations  are  to  be  expected  occasionally  would 
seem  to  be  indicated  by  the  fact  that  during  the  last  longer  (nine- 


116      SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

month)  period  of  the  experiment  the  mean  for  the  minus  strain  for  1 
two-month  period  (December  1916-January  1917)  fell  to  within  48 
seconds  of  that  for  the  plus  strain,  yet,  from  consideration  of  the 
data  for  this  line  for  the  21  months  at  the  end  of  the  experiment,  no 
one  would  be  inclined  to  question  the  validity  of  a  real  and  large 
difference  in  reactiveness  between  the  two  strains.  Further,  periods 
of  local  shifts  in  reactiveness — changes  which  are  believed  to  be 
purely  environmental  and  non-genetic — are  seen  in  other  lines,  e.  g., 
Lines  695  and  740  (see  pages  44  and  86),  in  which  there  is  not  evi- 
dence of  an  effect  of  selection.  The  approximation  of  the  curves 
for  the  plus  and  minus  strains  during  1914  and  again  in  1915 
seems  to  have  been  due  to  differential  local  environmental  influences 
(see  also  pages  140-142). 

There  is  an  additional  suggestive  feature  of  the  general  course 
of  the  curve  for  the  Line  757  selection  data.  It  is  in  the  marked 
general  lowering  of  the  mean  reaction-time  for  both  the  plus  and 
minus  strains  of  Line  757.  The  curve  starts  at  743  seconds  for  the 
plus  strain1  and  at  the  close  of  the  experiment  ends  at  124  seconds.2 

The  average  reaction-time  for  the  plus  strain  for  the  first  9 
months  of  selection  was  788  seconds  and  for  the  last  9  months  313 
seconds,  while  the  intervening  3  year-periods  of  data  show  a  pro- 
gressive and  consistent  lowering  of  this  reaction- time  as  follows: 
684,  605,  458.  The  form  of  the  curve  suggests  that  this  lowering 
had  not  ceased,  but  was  continuing  when  the  experiment  closed. 

The  curve  shows  a  slight  lowering  of  the  reaction-time  for  the 
757  minus  strain,  but  the  amount  is  small  and  the  mean  reaction- 
times  for  the  longer  periods  of  the  experiment  do  not  indicate  as 
large  and  consistent  lowering  in  this  as  with  the  plus  strain.  The 
means  for  the  minus  strain  for  the  longer  periods  are  839,  783,  707, 
770,  and  626  seconds. 

EXTENT  OP  THE  CHANGE  IN  REACTIVENESS  OF  THE  LINE  757  STRAINS. 

The  lowering  of  the  mean  reaction-time  for  the  757  plus  strain 
may  be  examined  further.  When  it  is  recalled  that  a  mean  reaction- 
time  of  788  seconds  (that  for  the  plus  strain  for  the  first  longer  period) 
means  that  by  far  the  greater  part  of  the  individuals  failed  to  react8 
to  the  light  stimulation  during  the  test  (actually  77.2  per  cent  of  the 
individuals  of  the  plus  strain  during  the  first  nine-month  period 
failed  to  reach  an  end  of  the  tank)  and  that  a  mean  reaction-time 
of  313  seconds  indicates  that  a  great  preponderance  of  the  individuals 
responded  to  the  light  stimulation  (only  15.1  per  cent,  less  than  one- 
tenth  as  many  as  during  the  earlier  period,  failed  to  respond  during 
the  final  nine-month  period)  the  decline  in  reaction-time  from  788 
seconds  to  313  seconds  takes  a  still  greater  significance.  Not  a 

1  Average  for  2^  months'  data,  88  individual  records. 

*  Average  for  1  month's  data,  42  individual  records. 

'That  is,  reach  an  end  of  the  tank  during  the  15-minute  period  of  the  test. 


A   PHYSIOLOGICAL   CHARACTER.  117 

single  brood  in  the  first  longer  period  failed  to  contain  individuals 
which  did  not  respond  during  the  15  minutes  to  the  test.  In  the 
final  longer  period  nearly  half  the  broods  tested  contained  no  over- 
time individuals.  Of  the  last  10  broods  tested,  only  3  contained 
over-time  individuals  and  the  last  5  broods  contained  none.  The 
change  in  reactiveness  in  this  strain  is  also  strikingly  indicated  by 
again  referring  to  the  average  minimum  reaction-times  in  the  first 
and  last  periods.  For  the  first  nine-month  period  it  was  468  seconds; 
in  the  last  period  94  seconds.  These  figures,  it  will  be  recalled,  refer 
only  to  the  most  reactive  individuals  of  each  brood,  but  when  these 
have  become  5  times  as  reactive  as  during  the  early  period  of  the 
experiment,  the  fact  can  not  fail  to  have  considerable  significance. 

The  change  in  responsiveness  in  the  plus  strain  of  Line  757  was 
in  no  other  way  as  striking  to  an  observer  as  the  change  in  the  general 
behavior  of  the  animals  in  the  experimental  tank.  Early  in  the 
history  of  this  strain  the  young  daphnids,  on  being  released  in  the 
experimental  tank,  ordinarily  behaved  as  follows:  first  they  settled 
to  the  bottom  in  a  rather  close  group;  after  3  to  5  minutes  a  few 
(usually  less  than  30  per  cent)  moved  a  little — ordinarily  toward  the 
source  of  light,  though  many  movements  were  indifferent  to  the 
light.  (In  about  a  third  of  the  earlier  selection  tests  of  entire  broods, 
there  was  no  movement  at  all  after  the  animals  settled  to  the  bottom 
of  the  tank,  though  these  were  vigorous  individuals.)  Of  the  small 
number  swimming  toward  the  light,  about  two-thirds,  in  spite  of 
many  pauses,  made  their  way  to  the  end  of  the  tank  within  the 
period  (15  minutes)  of  the  test.  Toward  the  close  of  the  selection 
experiment  with  Line  757,  the  behavior  of  the  757  plus  strain  was 
essentially  as  follows :  the  animals,  on  being  released,  ordinarily  settled 
to  the  bottom  as  previously,  but  within  a  few  seconds  some  of 
them  began  to  swim  toward  the  light  and  within  a  minute  or  two  all 
were  usually  under  way  toward  the  positive  end  of  the  tank,  which 
the  majority  reached  with  few  interruptions  of  their  swimming 
movements.  In  the  earlier  part  of  the  experiment  the  animals 
frequently  did  not  move  a  second  time  after  once  swimming  for 
a  moment  and  then  settling  to  the  bottom  or  holding  fast  to  the 
surface  film  or  the  sides  of  the  tank.  At  the  close  of  the  experiment 
the  movements  in  the  plus  strain  were  not  only  more  prompt  and 
general,  but  were  less  often  interrupted,  and  when  interrupted  were 
generally  resumed  rather  promptly. 

There  was  to  the  eye  of  the  observer  possibly  some  increase  in 
reactiveness  of  the  757  minus  strain  as  exhibited  in  its  general  be- 
havior, but  it  was  slight  as  compared  with  the  change  which  occurred 
in  the  757  plus  strain. 

The  lowered  mean  reaction-times  for  the  strains  of  757  during 
the  course  of  the  selection  experiment  are  in  partial  accord  with 
the  general  lowering  of  the  mean  which  occurred  for  all  of  the  strains 


118 


SELECTION   IN    CLADOCERA    ON   THE    BASIS   OF 


of  Simocephalus  during  the  course  of  the  selection  experiment.  This 
general  lowering  of  the  mean  is  probably  attributable  to  environ- 
mental conditions  (see  pages  137-1 39). l  But  in  the  case  of  the 
757  minus  strain,  and  particularly  during  the  final  11  months  of  the 
experiment,  the  mean  was  lowered  much  less  than  that  for  each  of 
the  other  9  selected  strains  of  this  species.  Selection  here  seems  to 
have  operated  to  hold  the  mean  much  higher  than  it  otherwise 
would  have  been. 

REACTIVENESS  OF  BOTH  STRAINS  OF  LINE  757  MODIFIED  THROUGH  SELECTION. 

Another  method  of  checking  up  the  effect  of  selection  in  Line 
757  is  by  a  detailed  comparison  of  the  mean  reaction-times  of  the 
plus  and  minus  strains  of  Line  757  with  corresponding  data  for 
Line  740,  with  which  it  is  most  similar  in  point  of  time  of  beginning 
the  selections  and  in  duration  of  the  selection  experiment;  and  by  a 
further  comparison  of  the  reaction-time  means  for  the  two  strains 
of  Line  757  with  corresponding  mean  reaction-times  for  the  plus  and 
the  minus  strains  of  the  other  lines  of  the  same  species,  Lines  794,  795, 
and  796. 

This  comparison  is  given  in  detail  in  tables  48  and  49  and  serves 
to  show  whether  both  the  plus  and  minus  strains  of  Line  757  became 
modified  during  the  selection  or  whether  the  divergence  between  the 
two  strains  of  this  line  resulted  from  a  genetic  change  in  reactiveness 
of  the  minus  strain  alone.  This  would  seem  to  have  an  important 
bearing  upon  the  question  as  to  whether  the  results  came  from 
mutation  or  gradual  change  in  Line  757.  It  would  seem  less  easy 
to  explain  the  result  as  due  to  larger  mutations  if  both  the  plus  and 
minus  strains  of  Line  757  were  affected,  i.  e.,  departed  from  the 
means  for  corresponding  strains  of  the  other  lines  of  the  same  species, 
than  if  only  one  of  the  two  strains  of  Line  757  was  affected. 

The  comparison  between  the  mean  reaction-times  of  the  Line  757 
plus  strain  and  the  plus  strains  of  the  other  S.  exspinosus  lines  is 
the  more  crucial  test,  for  it  is  obvious,  by  mere  inspection  and  com- 
parison of  the  figures  (figures  lie,  12c,  13c,  15,  and  18c)  showing  the 
reaction-time  curves  for  Line  757  and  the  other  S.  exspinosus  lines, 
that  the  Line  757  minus  strain  became  much  less  reactive  than  any 
of  the  other  S.  exspinosus  strains.  The  following  analysis  seems  to 
show,  however,  that  the  plus  strain  of  Line  757  likewise  became 
more  reactive  than  the  plus  strain  of  Line  740  or  than  the  plus 
strains  of  the  other  S.  exspinosus  lines. 

The  cyrves  showing  reaction-time  means  for  the  plus  strain  of 
Line  757  (figures  18s  and  19)  are  somewhat  irregular,  but  their 

1  Possibly  the  general  lowering  of  the  means  in  both  the  plus  and  minus  strains  of  all  the 
lines  of  S.  exspinosus  is  to  be  accounted  for  through  a  better  handling  of  the  material  as  the  exper- 
iment progressed.  No  intentional  changes  in  the  handling  of  the  material  were  made  after  about 
July  1912,  however,  and  we  were  not  conscious  of  any  better  manipulation  as  the  experiments 
progressed. 


A   PHYSIOLOGICAL   CHARACTER. 


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A   PHYSIOLOGICAL   CHARACTER,  123 

general  trend  is  very  nearly  that  of  a  straight  line.  This  general 
form  constitutes  intrinsic  evidence  of  the  reliability  of  the  curves 
as  a  whole. 

The  curve  for  Line  757  plus  (figure  15)  starts  higher  than  the 
level  attained  by  the  Line  740  plus  strain  for  the  same  period.  This 
is  probably  significant.  The  two-month  period  curve  for  the  Line  757 
plus  strain  held  very  closely  to  a  level  near  800  seconds  for  the  first  9 
months  of  selection.  The  curvje  for  the  plus  strain  of  Line  740, 
while  prevailingly  at  a  lower  level  for  these  periods,  fluctuated  from 
672  seconds  to  876  seconds.  The  mean  difference  between  the 
plus  strains  of  Lines  757  and  740  for  this  nine-month  period  is 
39  ±12.36  seconds,  3.15  times  the  probable  error.  Hence  there  is 
evidence  that  at  the  start  the  plus  strain  of  Line  757  was  less  re- 
active than  the  plus  strain  of  Line  740. 

During  the  first  31  months  (to  February  1915)  of  selection  with 
Line  757  the  plus  strain  in  general  had  a  higher  reaction-time  than 
the  plus  strain  of  Line  740  (table  48  and  figure  15),  which  tends  to 
strengthen  one's  conviction  that  at  the  start  the  plus  strain  of  Line 
757  was  less  reactive  than  the  plus  strain  of  Line  740.  But  from 
February  1915  the  fairly  consistent  differences  between  the  plus 
strains  of  Lines  757  and  740  are  striking  and  indicate  that  the  plus 
strain  of  Line  757  had  become  the  more  reactive  of  the  two.  Of  14 
two-month  periods  the  plus  strain  of  Line  740  was  the  more  reactive 
only  twice.  One  of  these  differences  is  slight  (16  ±41. 5  seconds), 
the  other  is  large  (246. 6 ±34. 2  seconds);  but  this  difference  arises 
from  the  coincidence  of  an  exceptionally  low  average  for  Line  740 
plus  (the  lowest,  by  104  seconds,  of  any  two-month  average  for  this 
strain)  and  an  exceptionally  high  average  for  Line  757  plus.1  Aver- 
ages by  two-month  periods  are  obtained  from  too  few  individual 
reaction-time  records  to  be  highly  reliable  when  considered  singly, 
so,  while  this  difference  is  large,  it  must  be  considered  in  relation  to 
the  other  averages  and  evaluated  accordingly.  For  the  other  12 
two-month  periods  of  the  latter  half  of  the  experiment  with  those 
two  lines,  the  plus  strain  of  Line  757  was  the  more  reactive  as  com- 
pared with  the  plus  strain  of  Line  740;  the  differences  were  from  22 
to  240  seconds,  and  were  respectively  3.6,  1.0,  0.7,  9.2,  1.6,  2.7, 
3.1,  2.6,  1.7,  3.7,  1.4,  and  6.4  times  their  probable  errors  (see  table 
45).  That  is  to  say,  even  when  considered  by  shorter  (two-month) 
periods,  12  of  14  differences  indicate  that  the  plus  strain  of  Line 
757  was  more  reactive  than  the  plus  strain  of  Line  740,  although, 
with  their  large  probable  errors  due  to  relatively  small  numbers, 
only  7  of  these  differences  are  statistically  significant. 

lrThe  mean  for  the  December  1916-January  1917  period  for  the  plus  strain  of  Line  757 
was  obtained  from  the  reaction-time  records  of  only  36  individuals  of  3  broods.  The  corre- 
sponding mean  for  the  740  plus  strain  resulted  from  41  individual  reaction-time  records.  These 
numbers  are  scarcely  more  than  half  the  usual  numbers  for  two-month  periods. 


124  SELECTION   IN    CLADOCERA    ON   THE    BASIS    OF 

Considered  by  longer  periods,  it  is  found  that  during  the  first 
(9  months)  and  second  (12  months)  longer  periods  of  the  experiment 
the  plus  strain  of  Line  757  was  less  reactive  than  the  Line  740  pus 
strain  by  39.0  ±12.36  seconds  and  86. 6  ±13. 11  seconds,  differences 
of  statistical  value,  3.15  and  6.6  times  the  probable  errors.  During 
the  following  year  the  plus  strain  of  Line  757  became  the  more  re- 
active, though  the  difference  for  the  entire  year-period  was  only 
37  ±13. 61  seconds,  or  2.76  times  the  probable  error.  During  the 
next  year,  August  1915-July  1916,  the  difference  is  116.4  ±13. 67 
seconds,  8.51  times  the  probable  error — the  plus  strain  of  Line 
757  being  the  more  reactive.  The  averages  for  the  final  nine- 
month  period  of  selection  differ  by  1.9  ±15. 89  seconds,  the  plus 
strain  of  Line  740  having  the  lower  reaction-time.  This  on  its  face 
distinctly  controverts  the  existence  of  a  significant  difference  in 
reactiveness  between  the  plus  strains  of  lines  757  and  740.  But,  as 
already  noted,  in  one  two-month  period  during  this  longer  period 
there  was  an  exceptionally  low  average  for  the  plus  strain  of  Line 
740  and  an  exceptionally  high  average  for  the  plus  strain  of  Line 
757,  and  the  striking  influence  of  these  two  aberrant  averages  pro- 
duces the  difference  noted  above,  a  difference  which  is  quite  out  of 
line  with  the  general  behavior  for  the  latter  part  of  this  experiment 
of  the  two  strains  concerned. 

The  curves  for  Lines  740  and  757  by  six-month  periods  (figure 
19)  show  the  plus  strain  of  Line  757  the  less  reactive  until  the  May- 
October  1915  periods,  from  Which  time  the  757  plus  strain  is  shown 
the  more  reactive  for  the  remainder  of  the  experiment.  The  ex- 
ceptional reaction-time  periods  (December  1916-January  1917)  for 
the  two  strains  compared  serve  to  cause  the  curves  to  approach  for 
the  last  six-month  period,  but  the  plus  strain  of  Line  757  maintains 
the  lower  level. 

Hence  it  is  seen  that,  whereas  on  the  start  the  plus  strain  of 
Line  757  was  fundamentally  less  reactive  than  the  plus  strain  of 
Line  740,  during  the  latter  half  (nearly)  of  the  experiment  there  is 
fairly  conclusive  evidence  that  the  plus  strain  of  Line  757  was  more 
reactive  than  the  plus  strain  of  Line  740,  which  may  fairly  be  con- 
sidered as  a  check,  since  it  was  subjected  to  the  same  treatment  as 
Line  757  and  throughout  a  contemporaneous  period  of  time. 

Comparing  the  mean  reaction-times  by  two-month  periods  for 
the  plus  strain  of  Line  757  with  the  means  for  the  plus  strains  of  the 
remaining  lines  of  S.  exspinosus  subjected  to  selection,  Lines  794, 
795,  and  796,  the  result  is  still  more  suggestive  of  a  changed  reactive- 
ness  of  the  plus  strain  of  Line  757.  The  remaining  3  lines  are  lumped 
in  this  comparison,  a  procedure  justified  only  in  the  interest  of 
brevity  and  the  securing  of  less  fluctuating  averages.  The  plus 
strain  of  Line  757  was  less  reactive  than  the  combined  plus  strains 
of  Lines  794,  795,  and  796  in  only  3  out  of  15  two-month  periods,  the 


A   PHYSIOLOGICAL   CHARACTER.  125 

means  for  Line  757  plus  being  exceptionally  high,  in  each  of  these 
periods.  For  the  other  12  periods  Line  757  plus  was  the  more  re- 
active, the  differences  varying  from  25  to  269  seconds  and  being 
4.1,  1.1,  1.8,  5.1,  14.0,  1.0,  2.0,  4.6,  3.1,  9.3,  5.6,  and  8.1  times  the 
large  statistical  probable  errors  (table  48).  Considered  by  longer 
periods,  it  is  seen  (table  48)  that  the  differences  are  -5.1  ±13. 97 
seconds,  +123.1  ±10.24  seconds,  and  -f-89.5±  13.86  seconds,  the 
plus  sign  indicating  greater  reactiveness  on  the  part  of  the  plus 
strain  of  Line  757.  Reference  to  figure  ISA,  which  shows  the  re- 
action-time curves  for  Line  757  and  combined  reaction-time  curves 
for  all  other  S.  exspinosus  lines,  Lines  740  (in  part),  794,  795,  and 
796,  shows  clearly  to  what  extent  the  mean  reaction-time  for  the 
plus  strain  of  Line  757  is  prevailingly  lower  than  the  combined  means 
for  all  the  other  S.  exspinosus  plus  strains.  These  curves  show  at  a 
glance  that  the  plus  strain  of  Line  757  was  essentially  lower  in  its 
reaction-time  than  the  other  plus  strains  of  the  same  species.  The 
3  two-month  periods  in  which  the  reverse  exists  are  there  seen  in 
their  true  light — as  exceptionally  high  points  in  the  reaction-time 
curve  for  757  plus. 

Thus,  from  examination  of  the  data  for  mean  reaction-times 
for  contemporaneous  periods,  it  is  found  that  during  the  latter  half 
of  the  experiment  the  plus  strain  of  Line  757  is  more  reactive  than 
the  plus  strains  of  the  other  S.  exspinosus  lines.  That  this  is  a  result 
of  selection  is  indicated  by  the  fact  that  in  the  beginning  of  the 
experiments  both  strains  of  Line  757  were  appreciably  less  reactive 
than  the  two  strains  of  Line  740,  with  which  Line  757  may  fairly  be 
compared. 

Comparison  of  the  reaction-time  means  for  the  minus  strain  of 
Line  757  with  the  740  minus  strain  (table  49  and  figures  15  and  19) 
shows  at  once  that  the  minus  strain  of  Line  757  was  from  the  be- 
ginning the  less  reactive  of  the  two,  and  that  as  the  experiment 
progressed  the  difference  in  reactiveness  between  the  minus  strains 
of  the  two  lines  increased  until  at  the  close  of  the  experiment  the 
divergence  was  fairly  large.1  Of  27  two-month  periods  the  minus 
strain  of  Line  757  had  a  lower  reaction-time  than  Line  740  minus 
only  2  times,  a  remarkable  uniformity  of  differences  when  the 
irregularities  of  the  curves  are  considered.  Of  the  25  times  when 
the  minus  strain  of  Line  757  was  the  less  reactive,  the  differences 
were  of  statistical  value  18  times,  being  4.5,  5.1,  3.4,  10.1,  3.0,  5.7, 
11.5,  6.5,  7.2,  3.3,  12.8,  9.7,  6.5,  4.4,  5.4,  5.7,  8.6,  and  12.3  times  their 
probable  errors  (see  table  49).  Seven  of  the  differences  by  shorter 
periods  and  the  2  differences  in  the  reverse  direction  were  not  of 
statistical  significance.  Considered  by  longer  periods,  the  mean 

1  As  is  general,  when  comparing  reaction-time  means  of  two  strains  for  contemporaneous 
periods,  there  is  seen  an  obvious  rough  parallelism  between  the  two  curves ;  but  the  uniformity 
with  which  these  two  curves  parallel  each  other  is  rather  exceptional. 


126  SELECTION    IN    CLADOCERA   ON   THE    BASIS    OF 

differences  were  77,  126,  99,  170,  and  230  seconds.  The  difference 
for  the  middle  period  is  influenced  by  the  exceptionally  low  means 
for  the  Line  757  minus  strain  during  2  two-month  periods.  Other- 
wise the  divergence  consistently  increased. 

When  the  minus  strain  of  757  is  compared  with  the  shorter  S. 
exspinosus  lines  (Lines  794,  795,  and  796)  the  divergence  between  the 
minus  strain  of  Line  757  and  the  minus  strains  of  other  lines  which 
may  serve  as  checks  is  even  more  marked  (table  49  and  figure  18s) . 
The  minus  strain  of  Line  757  had  a  higher  average  than  the  combined 
averages  for  the  minus  strains  of  Lines  794,  795,  796  in  14  of  15  two- 
month  periods,  the  differences  ranging  from  51  to  319  seconds  and 
all  but  two  of  them  being  from  4  to  13  times  their  probable  errors. 
The  one  exceptional  difference  was  a  difference  of  only  1.1  seconds. 

CONCLUSION  REGARDING  EFFECT  OF  SELECTION  IN  LINE  757. 

It  is  seen,  then,  in  Line  757,  that  by  all  the  tests  applied  and  in 
spite  of  all  the  safeguards  used  in  connection  with  these  data,  a 
marked  effect  of  selection  is  indicated.  This  is  shown  by  the  course 
of  the  curves  plotting  the  mean  reaction- times  of  the  two  strains;  by 
a  progressively  increased  (though  somewhat  irregular  and  once  inter- 
rupted) difference  in  mean  reaction-time  between  the  two  strains, 
the  plus  strain  having  the  lower  reaction-time;  by  a  decrease  to  94 
seconds  in  the  average  minimum  reaction-time  of  the  plus  strain  as 
compared  with  a  decrease  to  245  seconds  in  the  minus  strain;  by  a 
decrease  of  approximately  one-third  in  the  average  maximum  reac- 
tion-time in  the  plus  strain,1  while  at  the  same  time  the  minus  strain 
decreased  only  5  per  cent;  by  a  decrease  of  four-fifths  in  the  number 
of  individuals  of  the  plus  strain  which  failed  to  respond  to  the  light, 
while  the  decrease  in  this  number  in  the  minus  strain  was  only  one- 
third;  by  the  results  of  the  two  test  series  applied  to  the  strains  of 
this  line;  by  the  data  for  the  same-day  broods;  by  every  statistical 
test  which  was  applied  to  the  data;  by  a  comparison  with  similarly 
treated  strains  of  other  lines  of  the  same  species;  and,  to  the  eyes 
of  the  observer,  most  of  all  by  the  greatly  changed  behavior  in  the 
experimental  tank  of  the  plus  strain  as  compared  with  that  of  the 
minus  strain. 

There  seems  no  escape  from  the  conclusion  that  in  Line  757 
there  is  a  marked  effect  of  selection.  This  effect  appeared  in  the 
early  period  of  the  experiment  and  with  some  fluctuation  increased 
throughout  its  duration.  The  form  of  the  curves  would  seem  to 
indicate  that  the  effect  of  selection  was  continuous  and  was  still 

1This  decrease  in  the  maximum  reaction-time  means  more  than  a  decrease  of  one-third 
suggests.  At  the  beginning  of  selection  with  Line  757  some  broods  were  left  in  the  experimental 
tank  for  an  hour  or  more,  and  those  individuals  which  did  not  respond  to  the  light  within  the 
period  allotted  for  the  experiment  (15  minutes)  often  did  not  move  at  all  within  the  much  longer 
pi-rind.  It  seems  probable  that  the  maximum  reaction-times  for  this  early  period  might  readily 
have  been  near  3,600,  or  even  10,800  seconds  in  many  cases,  if  each  of  the  selection  tests  had 
been  continued  for  an  hour  or  3  hours. 


A   PHYSIOLOGICAL   CHARACTER.  127 

cumulative  at  the  close  of  the  experiment.  The  divergence  in  mean 
reaction-times  is  neither  controlled  nor  noticeably  influenced  by  the 
relative  vigor  of  the  two  strains,  nor  does  it  appear  to  be  explicable 
on  any  other  grounds  than  as  a  divergence  coming  about  through 
selection. 

It  is  to  be  regretted  that  no  return  selection  was  attempted 
with  Line  757.  The  selection  experiments  were  very  laborious,  and 
at  the  time  when  it  was  decided  that  they  had  been  carried  far 
enough,  there  did  not  seem  sufficient  justification  for  prolonging  the 
experiment.  Though  it  was  then  realized  that  the  two  strains  of 
Line  757  were  pronouncedly  different  in  their  reactiveness  to  light, 
the  full  significance  of  this  difference,  particularly  as  involving  a 
changed  reactiveness  in  both  strains  of  Line  757  as  compared  with 
the  corresponding  strains  of  the  other  S.  exspinosus  lines,  was  not 
brought  out  until  months  later,  when  the  extensive  data  were  fully 
worked  over.  Time  was  not  then  available  for  again  taking  up  the 
selection  experiment. 

A  further  fact  of  importance  is  that  the  effect  of  selection  was 
permanent,  or  at  any  rate  persisted,  through  a  long  series  of  genera- 
tions. In  December  1919  (32  months  after  selection  had  been  dis- 
continued) the  selective  effect  still  persisted  and  probably  to  as 
marked  a  degree  as  at  the  termination  of  selection.  The  two  strains, 
meantime,  had  been  propagated  for  112  generations  without  any 
regard  to  their  past  history.  The  persistence  of  the  effect  of  selection 
was  demonstrated  by  testing  a  large  number  of  broods  of  both 
strains.  In  extremely  few  cases  did  a  brood  of  the  plus  strain  fail 
to  show  more  reactiveness  to  light  than  the  corresponding  brood  of 
the  minus  strain,  while  in  most  cases  the  broods  of  the  plus  strain 
were  markedly  the  more  reactive. 


128 


SELECTION    IN    CLADOCERA    ON    THE    BASIS    OF 


ENVIRONMENTAL  CONDITIONS  AS  AFFECTING  REACTION- 
TIME  AND  VIGOR  OF  STOCK. 

Any  relation  between  environmental  influences  and  reactive- 
ness  to  light  is  a  fact  of  general  biological  significance  and  is  a  matter 
of  importance  in  the  present  connection.  In  the  light  tests  with 
Cladocera  little-understood  environmental  influences  were  important 
factors  in  influencing  reaction-time  in  all  three  species  which  were 
subjected  to  selection.  These  environmental  influences  are  dis- 
cussed under  the  following  heads:  (1)  temperature  influences;  (2) 
influences  of  exhaled  substances  from  the  observer's  breath;  (3) 
relatively  temporary  chemical  (?)  differences  in  the  water  used  in 
the  experimental  tank;  (4)  occurrence  of  negatively  reacting  indi- 
viduals; (5)  general  influences  operative  through  longer  periods  of 
time;  (6)  influence  of  vigor. 

1.  TEMPERATURE  INFLUENCES. 

The  temperature  of  the  water  in  the  experimental  tank  was  a 
minor  factor  influencing  reaction-time.  This  influence  was  not 
marked  enough  to  be  evident  to  the  observer  in  the  tests  day  by 
day,  but  the  somewhat  lower  reaction  means  during  the  winter 
periods  indicate  and  correlation  between  temperature  and  reaction- 
time  demonstrates  that  lower  temperatures  are  to  some  extent 
associated  with  greater  reactiveness  and  a  lower  reaction-time  mean. 

The  temperature  of  the  water  in  which  the  animals  were  tested 
was  always  near  that  of  the  basement  room  in  which  the  tests  were 
conducted.  Fortunately  this  room  was  less  affected  by  fluctuations 
in  summer  temperatures  than  the  remainder  of  the  building,  but  in 
winter  temperatures  ran  fairly  low,  -10°  to  15°  C.,  inasmuch  as 
this  room  was  not  provided  with  much  artificial  heat. 

Correlations  between  individual  reaction-times  (50-second 
classes)  and  temperature  (0.5°  C.  classes)  of  the  water  for  all  the 
tested  individuals  of  strains  of  D.  pulex  cover  the  following  periods 
and  give  the  following  values : 


Time 
period. 

No.  of 
tested 
young. 

r 

E 

rjW 

Time 
period. 

No.  of 
tested 
young. 

r 

ET 

r/ET 

Apr.  1914 

194 

+0.0071 

0.0484 

0.15 

1'V1>.  1915... 

292 

+0.0142 

0.0395 

0.36 

May  1914 

317 

-  .2107 

.0362 

5  .  82 

Mar.  1«.»15.. 

347 

-.0532 

.0361 

1.47 

Juno  1914 

175 

-  .0019 

.0610 

0.04 

(Summer) 

July   1914 

197 

-  .0009 

.0481 

0.02 

Apr.-Sept. 

AUK.  1914 

205 

+  .  1238 

.0464 

2.67 

r.'i  \  

1,474 

-.0193 

.0175 

1.10 

Sept.  1914 

386 

-.2482 

0322 

7.70 

(Winter) 

Oct.    1914 

880 

+  .2742 

0318 

8.62 

Oct.  1914- 

Nov.  1914 

188 

+  .()<)'.»;* 

.0821 

2.  it. 

Mar.  1915. 

2,025 

+  .0442 

.0150 

2.95 

I  ><•••.   1914 

301 

+  .086] 

.0386 

2.20 

(Year) 

Jan.    1915 

MS 

+  .0960 

.0419 

2.35 

Apr.  1914- 

Mar.  I'.il.'i. 

3,499 

+  .  1369 

.0112 

12.22 

A   PHYSIOLOGICAL    CHARACTER.  129 

These  correlations  are  small  and  of  those  for  single  month-periods 
only  three  or  four  can  be  credited  with  statistical  value,  those  for  the 
May,  August,  September,  and  October  1914  periods.  Two  of  these 
are  positive  and  two  are  negative  correlations.  Five  of  the  correla- 
tions from  March  to  September  are  negative  and  two  of  these  are 
significant,  both  in  numerical  value  and  in  comparison  with  their 
probable  errors.  One  of  the  two  positive  correlations  for  the  spring 
and  summer  period  is  very  small,  but  the  other  is  larger  and  of 
possible  significance.  The  correlations  for  October-February  are 
all  positive,  but  only  one  is  significant.  The  correlation  for  the  six- 
month  spring  and  summer  period  is  —0.0193  ±0.0175;  that  for  the 
six-month  winter  period  is  +0. 0442  db  0.0150.  These  very  limited 
data  suggest  that  negative  correlations  are  possibly  general  for  the 
spring  and  summer  months,  while  positive  correlations  are  general 
for  the  late  fall  and  winter  months.  The  correlation  for  the  year 
April  1914-March  1915,  while  only  +0.1369,  is  12.2  times  its  prob- 
able error. 

These  data  cover  the  year-period  of  most  consistent  shifts  in  the 
composite  reaction-time  curves  for  the  D.  pulex  strains  (figure  10D) 
and  probably  show  the  changes  in  reaction-time  associated  with 
season  and  temperature  at  its  maximum  for  D.  pulex. 

Correlations  between  temperature  and  reaction-time  for  S. 
exspinosus  were  confined  to  separate  correlations  for  the  strains  of 
Line  757  covering  in  each  case  the  selection  data  for  19  months  from 
October  1915  to  April  1917,  the  period  during  which  occurred  the 
greatest  differences  in  reaction-time  between  the  two  strains.  The 
correlations  were,  for  the  plus  strain  (689  individuals),  +0.2428  ±0.0242 
and  for  the  minus  strain  (683  individuals),  +0.2363  ±0.0244.  Each 
value  is  large  enough  to  have  a  real  meaning  and  is  approximately 
10  times  its  probable  error. 

Hence  there  is  shown  for  both  species,  and  particularly  for 
S.  exspinosus,  Line  757,  some  significant  correlation  between  reaction- 
time  and  the  temperature  of  the  water  in  the  experimental  tank.  It 
is  a  question  how  much  of  this  correlation,  if  any,  represents  a  real 
direct  temperature  influence.  If  the  temperature  influence  were 
direct,  one  would  perhaps  expect  within  the  limited  range  of  tempera- 
ture occurring  in  the  tests  that  higher  temperatures  would  be  asso- 
ciated with  greater  reactiveness  and  a  lowered  mean  reaction-time, 
which  would  be  indicated  by  negative  correlations.  Since  the 
significant  correlations  are  mostly  positive,  higher  temperatures  tend 
to  be  associated  with  less  reactiveness  and  a  higher  reaction- time. 

It  is  possible  that  the  lowered  reactiveness  with  higher  tempera- 
tures is  really  due  to  reduced  oxygen  in  the  water  used  in  the  experi- 
mental tank.  The  water,  when  brought  in  the  day  it  was  to  be  used, 
was  always  at  a  lower  temperature  than  the  experimental  room.  As 
it  warmed  up,  the  excess  gas  in  solution  appeared  as  bubbles.  The 


130  SELECTION    IN    CLADOCERA    ON   THE    BASIS    OF 

water  was  ordinarily  used  soon  after  it  reached  approximately  the 
room  temperature;  hence  it  was  probably  near  saturation  for  the 
principal  gases  which  had  been  thrown  out  of  solution.  Whether 
the  gases  retained  in  solution  were  equally  oxygen  and  other  gases 
would  of  course  depend  upon  the  actual  amounts  of  individual  gases 
within  the  water  when  taken  from  the  pond.  Assuming  that  the 
pond-water  was  ordinarily  near  saturation  for  oxygen  when  brought 
into  the  laboratory,  after  it  had  given  up  a  portion  of  its  contained 
gases  (as  it  always  did)  it  would  presumably  be  saturated  for  oxygen. 
Since  the  amount  of  oxygen  at  saturation  at  25°  C.  is  about  26  per 
cent  less  than  at  10°  C.,  this  is  a  basis  for  the  suggestion  of  an  ex- 
planation for  the  greater  reactiveness  at  the  lower  temperatures. 
No  specific  tests  have  been  made  to  determine  if  this  is  the  correct 
interpretation,  and  the  results  of  the  summer  correlations  for  D. 
pulex  do  not  fit  in  with  this  explanation. 

There  is  also  the  possibility  that  temperature  influenced  reaction- 
time  indirectly  through  its  effect  upon  the  food  in  the  culture-water. 
The  water  used  as  culture-water  went  through  the  changes  in  tem- 
perature (and  chemical  constitutents)  to  which  a  shallow  out-door 
pond  is  subject  in  the  climate  of  Cold  Spring  Harbor  and  the  develop- 
ment of  the  protista  upon  which  the  Cladocera  feed  is  naturally 
influenced  by  temperature  as  well  as  other  environmental  factors. 
Before  being  used  the  culture-water  (during  its  successive  strainings) 
was  kept  for  approximately  6  days  indoors  in  jars  surrounded  by 
running  water.  The  temperature  of  this  running  water  varied  with 
out-door  temperatures,  but  was  much  less  extreme.  Hence  there  is 
also  room  for  the  assumption  of  influences  of  temperature  upon  the 
Cladocera  stock,  not  directly  as  a  temperature  influence  per  se,  but 
indirectly  through  the  culture  medium,  which  is  a  veritable  micro- 
cosm. 

2.  INFLUENCE  OF  SUBSTANCES  EXHALED  FROM  OBSERVER'S  BREATH. 

That  the  amount  of  carbon  dioxide  was  actually  an  important 
factor  in  influencing  reaction-time  is  suggested  by  the  fact  that  if 
several  broods  were  tested  successively  in  the  experimental  tank 
without  change  of  water  the  later  broods  reacted  markedly  less 
promptly  than  the  ones  tested  in  the  same  water  an  hour  earlier.2 
The  young  Cladocera  are  so  small  as  to  be  just  visible  in  the  illumina- 
tion in  the  experimental  tank,  so  that  one  must  get  the  eyes  at  about 
reading  distance  from  the  water  of  the  tank  in  order  to  observe  the 
young  animals.  As  a  consequence,  the  exhaled  air  very  largely 

1  The  spring  water  is  known  to  contain  relatively  a  larce  amount  of  COj. 

2  This  was  observed  a  number  of  times,  after  which  the  plan  of  frequently  changing  the 
water  in  the  experimental  tank  was  adopted  to  obviate  this  disturbing  influence.     It  seemed 
impracticable  to  change  the  water  oftener  than  after  testing  each  four  broods,  although  it  was 
sometimes  noted  that  the  last  brood  or  two  of  the  four  tested  between  changes  of  water  was  less 
reactive  than  broods  tested  just  after  the  change. 


A   PHYSIOLOGICAL   CHARACTER.  131 

struck  the  surface  of  the  water.  These  differences  in  reactiveness 
were  not  due  to  temperature  differences,  since  they  occurred  in  cases 
where  there  were  no  temperature  differences  in  the  water.  It  is 
assumed  that  this  difference  in  reactiveness  was  due  to  the  accumu- 
lation of  carbon  dioxide  from  the  observer's  breath  in  the  water 
contained  in  the  tank,  though  exhaled  volatile  organic  substances 
may  have  been  wholly  or  partially  responsible. 

3.  RELATIVELY  TEMPORARY  CHEMICAL  DIFFERENCES  IN  WATER  USED  IN 

EXPERIMENTAL  TANK. 

Probably  the  environmental  factor  of  greatest  importance  con- 
sisted in  differences  in  the  chemical  content  of  the  water  used  in  the 
experimental  tank,  although  this  water  was  always  obtained  from  the 
same  pond  and  handled  in  the  same  fashion.  Such  chemical  differ- 
ences in  the  water  would  account  for  the  pecularities  in  reaction-time 
which  were  so  apparent  in  the  reactions  of  all  the  broods  tested  on 
certain  days.  Not  infrequently,  between  successive  days  or  be- 
tween periods  of  a  few  days'  duration,  considerable  difference  was 
noted  in  the  general  reactiveness  of  the  stock  as  a  whole.  On  one 
day  the  individuals  tested  would  react  fairly  promptly,  while  on  the 
following  day  they  might  react  conspicuously  less  promptly.  This 
clearly  points  to  environmental  influences  operative  temporarily  but 
pronouncedly.  These  temporary  differences  in  reactiveness,  pre- 
sumably due  to  differences  in  the  water,  are  well  illustrated  in  table 
2  (page  18),  showing  in  abbreviated  form  the  data  for  the  broods  of 
Line  695,  tested  on  August  29,  30,  and  31,  1913. x 

The  data  given  in  table  2  present  an  extreme  case  selected  to 
show  the  environmental  influence  upon  reaction-time  (and  upon  the 
occurrence  of  negatively  reacting  individuals)  at  its  maximum.  The 
averages  for  the  data  for  the  two  strains  of  Line  695  for  the  3  days 
shown  in  table  2  are  401,  636,  and  398  seconds,  based  upon  87,  190, 
and  50  individual  reaction-times.  The  mean  for  the  second  of  these 
days  is  235  seconds  or  58.6  per  cent  greater  than  that  for  the  first 
day  and  238  seconds  or  59.8  per  cent  greater  than  that  for  the  third 
day.  The  standard  deviations  and  probable  errors  for  these  means 
were  not  determined,  but  obviously  (from  other  experience  in  deter- 
mining probable  errors  with  such  data)  these  differences  are  of  large 
statistical  value.  As  already  noted,  this  is  an  extreme  case  (and  it  is 
doubtful  if  many  as  extreme  could  be  found  in  any  portion  of  the 
later  data),  but  the  factor  operative  here  presumably  was  also 
operative  to  some  extent  in  many  of  the  tests  conducted  in  making 

1  These  data  were  obtained  with  due  regard  to  the  necessity  for  changing  the  water  in  the 
experimental  tank  at  frequent  intervals  and  they  fit  the  requirements  of  the  present  purpose  very 
well.  They  are  part  of  the  data  of  the  first  test  series  conducted.  As  a  test  series  this  was  not 
satisfactory,  as  the  numbers  of  individuals  in  the  corresponding  plus  and  minus  broods  were  not 
as  nearly  exactly  the  same  as  in  later  test  series.  Also,  the  broods  were  not  always  paired  here, 
i.  e.,  in  the  usual  course  a  plus  brood  was  always  tested  just  before  or  immediately  after  a  minus 
brood  of  the  same  age  and  containing  the  same  number  (within  one  or  two)  of  individuals. 


132  SELECTION   IN    CLADOCERA    ON   THE    BASIS    OF 

selections;  but  this  effect  was  probably  the  same  with  all  the  indi- 
viduals of  the  plus  and  minus  strains  and  could  not  have  interfered 
with  the  choosing  of  the  proper  individuals  or  affected  the  experiment 
as  a  whole,  except  that  it  added  to  the  irregularity  of  the  reaction- 
time  records.  If,  however,  this  disturbing  influence  did  sometimes 
prevent  the  making  of  the  proper  selections,  that  would  only  render 
the  selections  less  effective  and  would  not  confuse  any  results  ob- 
tained. 

4.  OCCURRENCE  OF  NEGATIVELY  REACTING  INDIVIDUALS. 

The  data  shown  in  table  2  also  serve  to  illustrate  the  irregular 
occurrence  of  negatively  reacting  individuals.  There  were  3  negative 
ones  of  a  total  of  87  individuals,  or  3.5  per  cent,  August  29;  3  out  of 
190,  or  1.6  per  cent,  August  30;  and  14  of  50,  or  28  per  cent,  August  31. 
While  these  numbers  are  not  large,  the  widely  differing  percentages 
indicate  very  well  the  relatively  spasmodic  occurrence  of  negatively 
reacting  individuals  on  August  31  (see  also  page  15).  There  is  great 
irregularity  in  the  occurrence  of  negatively  reacting  individuals  in 
general,  so  that  while  the  illustration  taken  from  table  2  is  somehwat 
extreme,  it  is  not  unique.1 

These  illustrations,  (1)  to  (4),  bring  to  notice  relatively  tempo- 
rary disturbing  factors  which  are  a  matter  of  concern,  but  for  which 
no  way  of  complete  elimination  was  found;  yet  they  do  not  serve  to 
discount  the  selection  data  as  a  whole.  This  is  clear  for  two  reasons: 
(1)  these  factors,  which  in  the  long  run  presumably  must  have  affected 
the  plus  and  minus  strains  in  a  similar  way,  could  not  often  have 
interfered  with  making  the  proper  selections  and  could  contribute 
only  to  the  fluctuations  in  reaction-time;  (2)  the  consistent  and  con- 
vincing results  with  Line  757  could  not  have  come  about  if  these  dis- 
turbing factors  had  seriously  interfered  with  the  course  of  the  selec- 
tion experiment  as  a  whole. 

5.  GENERAL  INFLUENCES  OPERATIVE  THROUGH  LONGER  PERIODS  OF  TIME. 
The  general  relation  between  mean  reaction-time  and  environ- 
mental conditions  is  obvious  in  the  curves  plotting  mean  reaction- 
times  by  two-month  periods.  This  relation  is  seen  in  that  reaction- 
time  curves  for  the  plus  and  minus  strains  of  the  same  line  in  a  general 
way  move  upward  and  downward  together  to  some  extent  and  at 
times  the  nearly  coincidental  courses  of  the  curves  for  the  two  strains 
are  very  striking.  This  was  not  due  to  any  large  extent  to  tempera- 
ture differences  in  the  water  used  in  the  experimental  tank,  although 
temperature  differences  in  this  water  were  sometimes  a  factor  (see 
pages  128-130).  Temperature  influences  fail  to  explain  many  of  the 
broad  fluctuations  of  the  reaction-time  curves,  e.  g.,  the  high  levels 
obtaining  for  the  curves  for  D.  pulex  (figure  lOo)  during  December 

1  It  chances  in  this  case  that  the  lowest  percentage  of  negatively  reacting  individuals  falls 
upon  the  day  of  highest  mean  reaction-time.    This  is  a  mere  coincidence. 


A   PHYSIOLOGICAL   CHARACTER.  133 

1913-March  1914,  a  winter  period  during  which  relatively  low  levels 
are  generally  seen. 

It  is  not  difficult  to  believe  that  these  environmental  factors  may 
have  been  in  part  chemical  differences  in  the  pond-water,  gradual 
changes  in  carbon-dioxide  or  oxygen  content,  or  other  chemical 
differences,  since  the  effects  observed  are  operative  through  longer 
periods  of  time  than  the  temporary  environmental  effects  already 
considered.  Such  gradual  changes  in  the  water  of  a  pond  are  well 
known,  and  the  water  used  for  culture-water  and  in  the  experimental 
tank  during  these  tests  was  pond- water.  We  know  nothing  about  the 
precise  specific  character  of  the  changes  involved  in  the  present  case. 

COINCIDENT  CHANGES  IN  REACTIVENESS. 

The  curves  for  Line  695  (figure  2c)  show  these  general  environ- 
mental effects  to  a  marked  degree,  particularly  in  the  relatively 
low  levels  of  the  curves  for  October  1912-November  1913  and  in  the 
higher  level  from  December  1913-September  1914.  The  general 
coincidence  of  level  of  the  two  curves  does  not  end  here,  but  it  is 
not  so  marked  through  the  later  course  of  the  experiment  because 
of  the  relatively  wide  fluctuations  of  the  curve  for  the  minus  strain. 
The  correlation  between  the  mean  reaction-times  of  the  plus  and 
minus  strains  of  this  line  by  two-month  periods  is  0.4962  ±0.0979. 
With  Line  689  (figure  3c)  the  environmental  effect  is  observable 
throughout  the  course  of  the  curves.  In  Line  691  (figure  4),  while 
the  curve  for  the  minus  strain  is  quite  irregular,  the  general  course 
of  the  curves  is  very  similar.  The  same  may  be  said  for  Line  711 
(figure  5).  In  Line  713  (figure  6),  in  spite  of  the  very  irregular  curve 
for  the  plus  strain,  the  curves  in  general  follow  each  other  remark- 
ably closely.  The  correlation  for  the  two  strains  of  this  line  is 
0.6982  ±0.0773.  With  Line  714  (figure  7c)  the  curves  are  unusually 
irregular  for  strains  of  Daphnia  pulex,  yet  the  environmental  influence 
is  obvious  from  the  general  course  of  the  curves.  The  correlation 
is  0.5027  ±0.0971.  In  Line  719  (figure  8c),  again,  the  curve  for  the 
%plus  strain  is  very  irregular,  yet  the  general  coincidence  of  the  curves 
is  obvious. 

Line  751  (figure  9)  shows  the  same  general  coincidence  of  the 
curves.  This  line,  within  which  selection  was  begun  in  November 
1912,  is  the  only  D.  pulex  line  with  which  selection  was  begun  later 
than  March  1912.  It  is  interesting  to  note  that  the  curves  for  this 
line  start  at  virtually  the  same  level  as  was  attained  by  the  other  D. 
pulex  strains  during  the  same  two-month  period  (December  1912- 
January  1913)  and  closely  follow  the  composite  curves  for  the  other 
D.  pulex  strains  (see  figure  lOo). 

The  curves  (figure  10,  A  and  B)  for  the  relatively  short  exper- 
iments with  Lines  762  and  766,  both  Daphnia  longispina,  show  the 
same  general  indication  of  a  parallel  effect  of  environment  upon  the 


134       SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

mean  reaction-times  of  the  plus  and  minus  strains.  Likewise,  with 
the  other  D.  longispina  line,  Line  768,  the  curves  (figure  lOc)  for  the 
two  strains,  despite  irregularities,  follow  the  same  general  course. 

With  the  S.  exspinosus  material  the  general  influence  of  environ- 
ment upon  the  mean  reaction-times  of  the  two  strains  of  the  same 
line  is  less  conspicuous  than  with  the  lines  of  the  two  species  of 
Daphnia,  but  it  is  evident  in  most  of  the  curves  and  is  particularly 
clear  for  the  curves  for  the  final  several  months  of  the  experiments. 

The  curves  for  Line  794  (figure  lie)  show  this  influence  to  a 
marked  degree,  in  spite  of  the  irregular  portions  of  the  curve  for  the 
minus  strain.  Because  of  the  unusually  irregular  curve  in  Line  795 
(figure  12c),  the  environmental  influence  is  less  in  evidence,  but 
there  probably  is  a  small  positive  correlation  between  the  reaction- 
time  means  for  the  two  strains  by  the  two-month  periods  of  the 
experiment.  In  Line  796  (figure  13c)  the  curves  follow  each  other 
in  a  general  way. 

With  Line  740  (figure  15)  the  curves  on  the  whole  follow  each 
other  to  a  remarkable  degree.  A  clearer  demonstration  of  the  effect 
of  environmental  influence  working  through  relatively  long  periods 
would  be  hard  to  find.  The  correlation  between  the  mean  reac- 
tion-times for  the  plus  and  minus  strain  by  two-month  periods  is 
0.6437  db  0.0733.  The  relatively  shorter  range  of  the  up-and-down 
movements  of  the  curves  for  this  line  renders  this  correlation 
smaller  than  one  would  otherwise  expect  from  the  relatively  close 
parallelism  of  the  two  curves,  but,  considering  the  limited  num- 
ber of  terms  involved  in  the  correlation,  it  is  very  large. 

In  Line  757  (figure  18s)  the  environmental  effect  is  somewhat 
in  evidence  in  the  courses  of  the  two  curves,  but  naturally  it  is 
obscured  by  the  marked  effect  of  selection,  particularly  during  the 
last  21  months  of  the  experiment;  yet,  even  in  this  latter  period,  in 
spite  of  the  wide  divergence  of  the  curves,  there  is  an  evident  rela- 
tion between  the  two  which  seems  intelligible  only  as  an  effect  of 
environmental  influence.  The  correlation  for  the  reaction-time 
means  for  the  two  strains  is  0.4283  ±0.1041. 

A  composite  curve  plotting  the  mean  reaction-times  for  all  the 
plus  strains  and  another  for  all  the  minus  strains  of  D.  pulex  is  shown 
in  figure  10D.  The  courses  of  the  curves  for  the  strains  of  any  single 
D.  pulex  line,  despite  irregularities,  strikingly  resemble  the  corre- 
sponding portions  of  this  composite  curve.  Likewise,  the  few  re- 
action-time curves  for  strains  of  D.  longispina  (see  figure  10,  A,  B, 
and  c)  to  some  extent  follow  the  same  course  as  the  composite  re- 
action-time curve  (figure  IOD)  for  the  D.  pulex  strains. 

A  composite  curve  of  the  reaction-time  means  for  all  the  plus 
strains  and  one  for  all  the  minus  strains  of  S.  exspinosus  (except 
Line  757)  is  shown  in  figure  18B.  The  environmental  influence  is  well 
shown  in  the  generally  fairly  coincidental  courses  of  the  two  curves, 


A   PHYSIOLOGICAL   CHARACTER.  135 

and,  as  with  the  lines  of  D.  pulex,  there  is  a  striking  resemblance 
between  the  curves  for  any  one  of  the  strains  and  the  composite 
curve  for  all  the  corresponding  strains  of  S.  exspinosus. 

There  is  some  tendency  for  the  composite  reaction-time  curves 
for  strains  of  D.  pulex  to  follow  the  same  course  as  the  curves  for 
S.  exspinosus,  but  this  is  seen  to  a  limited  extent  and  in  only  a  few 
places,  such  as  in  a  general  rise  for  both  species  for  June- July  1913 
(figures  10D  and  15)  and  in  a  general  rise  followed  by  a  pronounced 
drop  in  the  period  from  April  to  November  1915  (figures  lOo  and  18). 
While  in  general  the  physiology  of  these  two  species  is  much  the  same, 
there  are  still  such  specific  physiological  differences  that  any  coinci- 
dental changes  in  reaction-times  bespeak  pronounced  contempo- 
raneous effects  of  environment. 

SEASONAL  CHANGES  IN  REACTIVENESS. 

There  is  some  tendency  observable  in  the  reaction-time  curves 
for  a  drop  in  the  curves,  indicating  greater  reactiveness  during  the 
winter  than  the  summer  months.  The  composite  curves  for  the  D. 
pulex  lines  (figure  lOo)  in  general  show  lower  portions  of  the  curves 
for  the  October-March  periods,  but  this  does  not  hold  for  the  winter 
period  for  1913-1914.  Seasonal  changes  are  seen  to  some  extent  for 
the  winter  and  summer  periods  in  the  composite  reaction-time 
curves  for  the  S.  exspinosus  lines  (figure  18s)  and  in  the  six-month 
curves  for  Lines  740  and  757  (figure  19). 

The  seasonal  (winter  and  summer)  differences  are  possibly  due 
largely  to  temperature  influences,  but  there  is  no  certainty  that  this 
seasonal  shift  is  due  to  temperature  alone  or  even  to  temperature  as 
indirectly  working  through  the  oxygen  or  other  content  of  the  water 
used  in  the  experimental  tank  and  through  the  culture  medium, 
yet  it  seems  probable  that  these  influences  may  be  a  large  factor  in 
these  seasonal  effects. 

CONTEMPORANEOUS  SHIFTS  IN  REACTION-TIME  MEANS. 

Contemporaneous  shifts  in  the  reaction-time  curves  for  the 
strains  of  different  lines  of  the  same  species  are  seen.  There  are  a 
few  shorter  time-periods  in  the  data  for  which  there  are  really  re- 
markable contemporaneous  changes  in  the  curves  for  the  different 
strains  of  the  same  species.  For  the  August-September  1912  period 
the  14  D.  pulex  strains,  with  two  exceptions,  showed  drops  in  the 
reaction-time  curves.  All  but  one  of  these  drops  were  considerable 
in  extent.  The  two  strains  which  did  not  show  drops  in  this  period 
and  the  one  which  showed  a  slight  drop  in  this  period  showed  pro- 
found drops  in  the  next  two-month  period,  while  most  of  the  strains 
showed  further  drops  in  this  later  two-month  period.  Temperature 
influences1  were  not  the  factors  determining  these  drops  in  the 

temperature  is  referred  to  frequently  because  the  data  include  temperature  records,  whib 
for  the  other  environmental  factors  no  definite  measurements  are  available. 


136  SELECTION    IN    CLADOCERA    ON   THE    BASIS    OF 

curves.  In  general,  the  temperature  of  the  water  in  the  experimental 
tank  was  very  much  the  same  for  August-September  1912  as  for 
June- July  preceding.  It  is  interesting  to  note  that  this  August- 
September  period,  because  of  poor  food  conditions,  was  a  period  of 
great  difficulty  in  maintaining  our  stock;  mortality  was  high;  most 
broods  were  excessively  small,  usually  1  to  3  individuals;  and  sterility 
was  relatively  common.  Yet  this  is  one  of  the  periods  of  the  greatest 
drops  in  the  curves  (decrease  in  reaction-time)  which  occurred  at  any 
part  of  the  experiment  with  D.  pulex. 

A  second  interesting  contemporaneous  change  in  the  reaction- 
time  curves  was  a  marked  rise  for  all  of  the  ten  strains  of  D.  pulex 
for  December  1913  to  January  1914.  Though  the  temperatures  were 
somewhat  lower  for  this  period  than  for  the  preceding-  period,  it 
should  be  noted  that  the  reaction-time  curves  for  D.  pulex  strains 
remained  very  close  to  this  high  level  for  8  months,  namely,  from 
December  1913  to  July  1914  (from  early  winter  to  midsummer). 
Hence  it  seems  impossible  to  attribute  this  fluctuation  in  the  D. 
pulex  curves  for  the  December  1913-January  1914  period  to  tem- 
perature influences. 

Another  local  drop  is  seen  in  every  one  of  the  10  curves  for  S. 
exspinosus  strains  for  the  February-March  1915  period.  This  drop 
is  very  large  for  5  of  these  strains.  Temperatures  for  this  two- 
month  period  were  in  general  slightly  lower  than  during  the  preceding 
and  succeeding  periods,  but  as  compared  with  other  periods  these 
are  comparatively  slight  temperature  differences  and  the  drops  in 
the  curves  can  not  be  considerd  primarily,  if  at  all,  due  to  the  slight 
temperature  differences  between  these  two  periods. 

It  is  worthy  of  note  that  the  contemporaneous  shift  in  the  re- 
action-time curves  affecting  every  one  of  the  10  strains  of  D.  pulex 
for  December  1913-January  1914  period  is  not  markedly  reflected 
in  the  4  S.  exspinosus  strains  (see  figures  15  and  18s)  and  that  the 
profound  drop  in  the  curves  for  strains  of  Simocephalus  for  the  last 
period  mentioned  above  is  not  in  evidence  for  the  strains  of  D.  pulex. 
This  in  itself  would  seem  to  indicate  clearly  enough  that  the  effect 
is  not  due  to  temperature  influences  peruse.1 

One  might  be  tempted  to  assume  that  the  first  and  third  of 
these  profound  shifts  in  the  reaction-time  curves  were  due  to  some 
improvement  in  Cladocera  stock  due  to  favorable  laboratory  condi- 
tions, inasmuch  as  the  first  appears  after  4  months  of  selection  in  the 
D.  pulex  lines  and  the  third  after  2  months  of  selection  in  3  of  the  S. 
exspinosus  lines,  except  for  the  facts  that  (1)  the  D.  pulex  lines  had 
been  under  laboratory  conditions  for  3J^  months  before  selection 
began  and  hence  a  total  of  7J^  months  before  these  drops  occurred, 
and  further,  that  they  did  not  maintain  these  lower  levels;  (2)  that 

1  The  lack  of  contemporaneous  shifts  in  the  curves  for  strains  of  D.  pulex  and  S.  exspinosua 
is  in  line  with  physiological  differences  between  the  two  species  observed  in  other  connections. 


A   PHYSIOLOGICAL   CHARACTER.  137 

Lines  740  and  757  had  been  in  the  laboratory  for  more  than  2  years 
before  these  fluctuations  occurred  in  the  8.  exspinosus  lines;  (3) 
that  no  such  improvement  is  shown  in  the  reproductive  indices.1 

While  with  the  older  lines  of  D.  pulex  subjected  to  selection  the 
reaction-time  means  decreased  markedly  almost  from  the  beginning 
of  selection,  and  the  means  for  these  lines  reached  a  low  point  in 
the  curves  (see  figures  10D,2c,  3c,4,  5,  6,  7c,  and  8c)  within  8  months, 
they  afterward  fluctuated  upward  and  downward  through  the  same 
general  range,  so  that  there  is  no  ground  for  assuming  that  this  early 
fluctuation  was  anything  other  than  one  of  the  wide  general  fluctua- 
tions of  the  curves.  This  decrease  in  mean  reaction-time  was  due 
to  environmental  conditions,  i.  e.,  factors  in  the  environment  which 
made  for  changed  conditions  under  which  the  animals  were  tested 
in  the  experimental  tank. 

Further,  comparison  of  the  curves  for  mean  reaction-times  for 
Line  751  (figure  9)  with  the  curves  for  the  other  D.  pulex  lines  shows 
that  the  reduced  reaction-time  means,  which  occurred  with  all  the 
older  D.  pulex  lines  simultaneously  soon  after  selection  was  begun, 
were  not  attributable  to  any  improvement  in  the  general  vigor  or 
quality  of  the  daphnid  stock  due  to  its  possibly  being  subjected  to 
more  advantageous  conditions  in  the  laboratory  than  in  the  ponds 
from  which  the  stock  was  obtained.  This  is  clear  because  the  curves 
for  the  two  strains  of  Line  751  not  only  do  not  show  a  drop  soon  after 
laboratory  culture  was  begun,  but  start  at  very  nearly  the  same 
level  as  was  attained  during  the  same  months  (December  1912- 
January  1913)  by  the  other  D.  pulex  lines  (figures  2  to  8)  which  had 
previously  been  subjected  to  laboratory  culture  for  about  11  months 
and  selection  for  approximately  7  months;  and  these  curves  for  Line 
751  follow  the  same  general  course  as  those  for  the  older  lines  (during 
the  same  two-month  periods)  regardless  of  the  fact  that  Line  751 
had  been  under  laboratory  culture  for  a  period  11  months  shorter 
than  that  for  the  older  lines. 

GENERAL  INCREASE  IN  REACTIVENESS  OF  SIMOCEPHALUS  EXSPINOSUS. 

With  the  S.  exspinosus  lines,  Lines  740  and  757, 2  there  is  a 
slight  simultaneous  increase  in  reactiveness  of  both  the  plus  and  the 
minus  strains  beginning  after  the  June- July  1913  period  (figure  15) 
and  continuing,  with  fluctuations,  throughout  the  rest  of  the  exper- 
iment. This  increase  is  more  marked  after  the  August-September 
1915  period  (figures' 18s,  lie,  12c,  13c,  and  15),  where  it  appears 
in  all  the  10  S.  exspinosus  strains,  though  to  a  less  degree  in  the 
minus  strain  of  Line  757.  The  curves  for  S.  exspinosus  strains 

1  Not  only  is  there  no  improvement  reflected  in  the  reproductive  indices,  but  those  indices 
for  the  D.  pulex  lines  are  abnormally  low  (indicating  reduced  vigor)  for  the  very  months  (in  the 
autumn  of  1912)  during  which  this  increase  in  reactiveness  is  most  marked. 

2  No  other  Simocephalus  lines  were  then  in  the  laboratory.    Lines  794,  795,  and  796  were 
begun  in  December  1914. 


138       SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

previous  to  August-September  1913  period  show  fairly  consistent 
rises  in  reaction-time,  but  henceforth,  in  spite  of  fluctuations,  some- 
what lower  levels  are  usually  maintained. 

It  is  extremely  improbable  that  these  general  increases  in  re- 
activeness  in  the  S.  exspinosus  strains  are  due  to  improvement  in 
the  stock  due  to  better  conditions  in  the  laboratory  than  in  the  out- 
door ponds  from  which  the  material  was  obtained.  The  increased 
reactiveness  does  not  appear  soon  enough  after  selection  was  begun 
with  Lines  740  and  757  to  render  this  interpretation  probable,  and 
there  was  an  actual  decrease  in  reactiveness  for  approximately  12 
months  (Line  740)  and  9  months  (Line  757)  after  laboratory  culture 
was  begun.  Then,  too,  the  increases  in  reactiveness  for  Lines  740 
and  757  are  simultaneous,  in  spite  of  the  fact  that  Line  740  had  been 
in  the  laboratory  for  about  12  months  and  Line  757  for  only  9  months. 
Further,  Lines  794,  795,  and  796  were  in  the  laboratory  for  10 
months  before  a  pronounced  general  tendency  to  increased  reactive- 
ness  is  evident  (figure  18s). 

Again,  with  the  3  lines  of  Simocephalus  (Lines  794,  795,  and 
796),  with  which  selection  was  begun  more  than  2  years  later  than 
with  the  older  S.  exspinosus  lines  (Lines  740  and  757),  the  reaction- 
time  curves  (figures  lie,  12c,  and  13c)  start  at  near  the  same  levels 
as  were  attained  by  the  two  older  lines  (figures  15  and  18s)  for  the 
same  months,  regardless  of  the  fact  that  the  older  lines  had  already 
lived  under  laboratory  conditions  and  undergone  selection  for  ap- 
proximately 28  and  25  months.1  The  striking  decrease  in  reaction- 
time  means  which  occurred  during  the  last  few  months  of  selection 
(except  to  a  lesser  degree  in  the  minus  strain  of  Line  757)  is  likewise 
as  pronounced  with  the  newer  as  with  the  older  S.  exspinosus  lines. 

A  further  and  (to  the  writer's  mind)  a  fatal  criticism  to  the 
assumption  (to  explain  decreased  reaction-time)  of  a  progressive 
improvement  in  the  stock  due  to  favorable  laboratory  conditions 
lies  in  the  fact  that  there  are  not  changes  in  reproductive  indices 
coincident  with  changes  in  reaction-time  and  that  there  is  no  pro- 
gressive increase  in  reproductive  index. 

To  explain  the  result  within  Line  757  as  due  to  a  differential 
physiological  improvement  requires  several  assumptions:  (1)  that 
such  improvement  is  not  shown  in  the  reproductive  index;  (2)  that 
Line  757  was  more  susceptible  to  such  improvement  than  the  other 
S.  exspinosus  lines;  (3)  that  not  only  was  the  plus  strain  of  Line 
757  subjected  to  greater  improvement  than  the  minus  strain,  but  to 
greater  improvement  than  the  other  S.  exspinosus  plus  strains;  (4) 

1  Omitting  the  minus  strain  of  Line  757 — which  because  of  the  effect  of  selection  had  a 
considerably  higher  mean  for  this  period,  829  seconds  (47  individual  records) — the  mean  re- 
action-time for  the  period  December  11,  1914,  to  January  15,  1915,  for  the  older  S.  exspinosus 
strains  was  645  seconds  (124  individual  reaction-time  records).  For  the  same  period,  the  period 
during  which  the  selection  was  begun  with  the  newer  lines  of  S.  exspinosus,  the  mean  for  the 
newer  lines  was  658  seconds  (319  individual  records). 


A   PHYSIOLOGICAL   CHARACTER.  139 

that  the  minus  strain  of  Line  757  through  the  selection  was  very 
little,  though  somewhat  improved;  (5)  that  this  improvement  was 
permanent,  or  at  least  persisted  for  32  months  after  the  close  of 
selection. 

The  writer  challenges  the  first  of  these  assumptions  as  contrary 
to  fact.1  Further,  at  no  time  was  there  any  observable  difference 
in  the  general  activity  of  the  two  strains  of  Line  757,  except  when 
subjected  to  directive  light  stimulation  in  the  experimental  tank. 
To  the  second,  third,  and  fourth  assumptions  the  writer  makes  no 
objection,  except  the  general  objection  that  there  is  no  obvious  ad- 
vantage derived  through  them  and  they  seem  uncalled  for;  further, 
there  is  no  obvious  reason  for  assuming  an  obscure  physiological 
effect  rather  than  genetic  changes.  The  last  of  these  assumptions 
appears  tantamount  to  the  assumption  of  a  permanent  extra-chro- 
mosomal inheritance  or  at  least  an  extra-genetic  transmission — an 
extremely  precarious  assumption. 

It  is  possible  to  assume  that  the  increase  in  reactiveness  as  the 
experiment  progressed  was  due  to  a  better  handling  of  the  material, 
such  as  to  affect  the  reactiveness  (but  not  the  reproductive  index),  but 
the  author  was  not  conscious  of  any  better  manipulation. 

The  logical  explanation  would  seem  to  be  that  the  pond-water 
which  was  used  in  testing  these  animals  was  undergoing  a  progressive 
change.  This  pond  is  fed  by  a  spring  and  has  considerable  overflow. 
It  is  a  dying  pond,  rapidly  filling  with  silt.  At  the  close  of  the  experi- 
ment the  spring-water  was  flowing  through  a  pond  which  then  had 
probably  less  than  one-third  the  water-content  which  it  had  at  the 
beginning  of  the  experiment.  The  water  was  always  taken  at  the 
outlet.  Consequently,  as  the  experiment  progressed,  the  water  used 
in  the  experimental  tank  had  remained  for  shorter  and  shorter  periods 
of  time  in  the  pond  from  which  it  was  obtained.  It  is  reasonable  to 
suppose  that  this  water  was  progressively  changed  in  some  of  its 
constituents  (carbon  dioxide,  oxygen,  substances  obtained  from  the 
silt,  or  in  other  constituents).  This  explanation  also  seems  plausible 
in  view  of  the  fact  that  reaction-time  is  clearly  closely  related  to  the 
condition  of  the  water  in  which  the  young  daphnids  are  tested.2 

Whatever  may  have  been  the  cause  for  the  generally  reduced 
reaction-time  for  S.  exspinosus  strains  during  the  latter  part  of  the 
experiment,  it  should  be  borne  in  mind  that  except  for  the  practice 
.of  selection  all  strains  were  treated  as  nearly  alike  as  possible  and 
that  there  is  apparently  no  ground  for  the  assumption  of  a  possible 
continued  differential  effect  due  to  selection  other  than  such  as  came 
about  through  genetic  changes. 

1  The  validity  of  the  reproductive  index  as  a  measure  of  vigor  (including  general  muscular 
activity)  is  discussed  on  page  143. 

2  Local  influences,  presumably  due  to  changes  in  the  water,  have  been  frequently  referred  to 
(see  table  2,  pages  17-18,  and  elsewhere). 


140  SELECTION    IN    CLADOCERA    ON    THE    BASIS    OF 

INDEPENDENT  SHIFTS  IN  REACTION-TIME  MEANS. 

Throughout  this  experiment  every  effort  was  made  to  treat  alike 
all  the  lines,  and  particularly  the  two  strains  of  a  line,  as  regards  food, 
temperature,  light,  method  of  handling  stock,  making  of  transfers, 
etc.  However,  the  two  strains  of  the  same  line  did  not  usually 
reproduce  on  the  same  day,  and  consequently  transfers  were  generally 
made  into  culture-water  of  different  collections  from  the  outdoor 
pond.  Such  variations  as  occurred  in  food  and  chemical  content  of 
the  culture- water,  and  temperature,  light,  etc.,  in  the  laboratory, 
impinged  upon  different  periods  of  the  life  of  the  new  generations. 
Such  unavoidable  differential  treatment  can  scarcely  be  conceived  to 
be  without  influence  upon  the  physiology  of  these  organisms.1 

In  spite  of  the  somewhat  differential  treatment  of  the  two  strains 
of  a  line,  there  has  been  seen  in  general  a  fairly  close  correlation 
between  the  reaction-time  means  for  the  two  strains  of  most  of  the 
lines  throughout  their  entire  history.  In  so  far  as  there  are  differ- 
ences in  reactiveness  between  the  two  strains  of  the  same  line,  where 
they  are  temporary  and  hence  are  presumably  non-genetic,  they  are 
believed  to  be  due  to  this  unavoidable  differential  environmental 
treatment. 

Pronounced  independent  shifts  in  the  reaction-time  curves  cover- 
ing considerable  periods  for  the  two  strains  of  the  same  line  are  seen  in 
several  cases.  Three  such  shifts  relative  to  each  other  are  seen  in  the 
reaction-time  curves  for  the  two  strains  of  Line  695:  (1)  September 
1912  to  January  1913,  in  which  the  minus  strain  is  consistently  higher 
than  the  plus  strain  and  higher  than  the  composite  curve  (figure  10o) 
for  the  minus  D.  pulex  strains  for  this  period;  (2)  September  1914  to 
May  1915,  in  which  the  minus  strain  is  consistently,  though  only 
slightly,  lower  than  the  plus  strain  and  lower  than  the  composite 
curve  for  the  period;  (3)  February-September  1916,  during  which  the 
two  strains  differed  consistently  by  a  widening  margin  due  to  the 
plus  curve  running  sharply  upward.  None  of  these  three  periods 
of  consistent  differences  in  reactiveness  for  periods  of  6  months  or 
longer  are  considered  to  have  genetic  significance  (see  pages  44-45). 

Two  shifts  are  seen  in  the  curves  for  Line  691,  September  1912- 
January  1913,  during  which  the  plus  curve  runs  lower  than  the  minus 
curve  and  lower  than  the  composite  curve  (figure  lOo),  and  June- 
September  1913,  during  which  the  minus  curve  runs  abnormally  high. 
This  last  shift  may  have  genetic  significance,  since  the  minus  curve 
tends  to  run  slightly  higher  than  the  Line  691  plus  and  the  composite 
curves  during  the  remainder  of  the  experiment,  but  its  genetic  sig- 
nificance is  extremely  doubtful. 

In  Line  713  the  plus-strain  curve  is  particularly  fluctuating, 
there  being  3  main  periods  of  differences  in  reaction-time:  December 

1  It  is  obvious  that  temperature  and  character  of  the  food  affect  the  reproductive  index. 


A    PHYSIOLOGICAL   CHARACTER.  141 

1912-October  1913,  when  in  general  the  plus  curve  was  low  (see 
figures  6  and  lOo);  December  1913- July  1914,  when  the  plus  curve 
was  relatively  high;  and  October  1914- July  1915,  when  the  plus 
curve  again  was  relatively  low,  compared  in  each  case  with  the  com- 
posite curve  (figure  lOo).  The  minus  curve  fluctuated  similarly,  but 
to  a  less  degree. 

In  Line  714  the  minus  curve  shows  3  parts  where  it  is  quite 
separated  from  the  plus  curve  (figure  7c) :  (1)  April-September  1912, 
when  it  is  high;  (2)  June-September  1913,  when  it  was  low;  April- 
July  1914,  when  it  was  high.  None  of  these  cases  has  apparent 
genetic  significance. 

Line  794  shows  (figure  lie)  2  fluctuations,  each  consisting  of  3 
two-month  periods.  During  the  first  (December  1914-May  1915) 
the  minus  strain  was  low,  and  during  the  second  (June-November 
1915)  it  was  high,  both  relative  to  the  plus  strain  of  Line  794  and  the 
composite  curves  for  the  other  S.  exspinosus  strains  (figure  18s). 

Line  795  shows  two  similar  periods  for  the  minus  strain  (figure 
12c):  February-May  1915  and  December  1915-July  1916,  during 
which  this  strain  was  lower  relative  to  the  plus  strain  of  this  line  and 
the  composite  curves  for  the  other  S.  exspinosus  strains  (figure  18s). 

Line  740  shows  3  periods  of  6,  10,  and  10  months,  during  which 
the  two  strains  differed  consistently  in  reaction-time.  The  first  2 
periods,  February  1914-July  1914  and  September  1914-May  1915, 
are  apparently  due  principally  to  fluctuations  in  the  plus  strain. 
There  were  no  sufficient  other  strains  of  this  species  in  the  laboratory 
during  this  time  to  make  adequate  comparisons.  During  a  third 
period,  April  1916- January  1917,  the  minus  strain  ran  higher  than 
the  composite  curve  for  the  other  minus  strains  (except  Line  757,  see 
figure  18fi).  For  the  latter  part  of  this  period  the  plus  strain  of  Line 
740  ran  abnormally  low.  The  two  fluctuations  produced  a  relatively 
large  difference  between  the  reaction-time  curves  for  the  two  strains 
of  this  line.  The  later  history  of  the  two  strains,  especially  the 
test-series  data  obtained  at  the  close  of  selection  with  this  line,  indi- 
cated that  this  divergence  had  no  genetic  basis. 

Line  757  shows  a  period,  June  1914-July  1915,  in  which  4  of  the 
7  two-month  periods  show  relatively  low  points  in  the  curve  for  Line 
757  minus  (figure  18s).  While  the  effect  of  selection  within  this  line 
is  such  that  the  minus  curve  drops  below  the  plus  curve  at  only  one 
point,  it  is  believed  this  clearly  represents  a  period,  or  two  periods 
(the  October  19 14- January  1915  period  shows  the  minus  curve  rela- 
tively high),  which  are  in  every  way  similar  to  these  just  referred  to 
(for  the  other  lines)  and  that  non-genetic  influences  served  to  mask 
the  effects  of  selection  during  this  portion  of  the  experiment. 

The  outstanding  fact  regarding  local  fluctuations  in  reaction- 
time  means  of  the  two  strains  in  Line  757  is  that  they  produce 
relatively  so  little  reduction  or  interruption  of  the  difference  between 


142       SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

those  means.  This  fact  would  seem  a  further  confirmation  of  the 
effect  of  selection  within  Line  757. 

Fluctuations  for  single  two-month  periods  are  common  in  the 
data.  They  are  presumably  due  to  the  same  factors  which  produce 
the  fluctuations  in  reaction-time  for  longer  periods.  It  is  also  true 
that  the  number  of  reaction-time  records  for  single  two-month 
periods  is  not  sufficient  to  obtain  highly  reliable  averages,  and  hence 
fluctuations  for  single  two-month  periods  are  less  significant  than 
those  covering  3  or  more  such  periods. 

Purely  local  differences  in  reactiveness  might  be  expected  to  be 
seen  occasionally  in  results  obtained  from  the  different  test  series, 
and  such  proves  to  be  the  case.  The  first  test  series  for  Line  695 
gave  a  mean  reaction-time  for  the  plus  strain  32  seconds  greater  than 
that  for  the  minus  strain.  The  second  test  series  (9  months  later) 
gave  a  mean  for  the  minus  strain  62  seconds  the  greater.  The  other 
2  test  series  for  Line  695  were  in  consecutive  months,  yet  the  differ- 
ence in  one  was  —  1  second  and  the  other  -f-51  seconds.  Such  local 
differences  lend  caution  to  one's  interpretation  of  the  result  of  test 
series,  and  the  results  of  test  series  are  given  credit  only  when  they 
show  pronounced  agreement  and  there  is  other  confirmatory  evidence. 

In  the  first  test  series  for  Line  757  the  mean  for  the  plus  strain  is 
282  seconds  lower  than  the  mean  for  the  selection  data  for  the  three- 
month  period  during  which  this  test  series  was  conducted  (table  45) ; 
while  during  the  second  test  series  the  mean  for  the  plus  strain  was 
133  seconds  higher  than  that  for  the  selection  data  for  the  contingent 
three-month  period.  It  is  extremely  probable  that  if  another  test 
series  had  been  conducted  within  a  few  weeks  after  the  first  one,  the 
mean  for  the  plus  strain  would  have  been  more  nearly  within  the 
range  of  the  selection  data  for  the  plus  strain  for  that  period,  and  that 
another  test  series  conducted  a  few  weeks  after  the  second  test  series 
would  have  given  a  much  lower  mean  for  the  Line  757  plus  strain. 
The  reaction-time  differences  between  the  two  strains  of  Line  757  are 
so  large,  however,  that  a  wide  departure  from  the  usual  reactiveness 
of  a  strain  still  leaves  a  large  difference  in  mean  reactiveness  between 
the  two  strains.1 

WHETHER  "DEPRESSION  PERIODS"  OCCUR. 

These  periods  of  relatively  temporary  drops  or  rises  in  the 
reaction-time  curves  are  probably  in  no  case  due  to  genetic  influences. 
In  cases  in  which  the  reaction-time  curves  rise,  the  large  and  some- 
what continuous  lifts  in  the  level  of  the  curves  may  be  ascribed  by 

1  For  the  benefit  of  any  who  (in  view  of  the  fact  that  local  environmental  effects  are  seen 
even  in  test  series)  might  be  inclined  to  wonder  if  additional  test  series  would  have  shown  such 
large  divergences  between  the  two  strains  of  Line  757,  it  may  be  added  that  in  July  1919  and 
again  in  December  1919  (27  and  32  months  after  selection  was  discontinued)  extensive  test  series 
were  conducted  in  Line  757,  each  covering  a  period  of  and  consisting  of  material  from  three 
generations.  In  extremely  few  cases  in  these  large  series  did  a  brood  of  the  plus  strain  fail  to 
show  markedly  greater  reactiveness  than  the  corresponding  brood  of  the  minus  strain. 


A   PHYSIOLOGICAL   CHARACTER.  143 

some  workers  to  "  depression  periods  "  in  the  Cladocera  stock.     This 
explanation  is  rejected  for  the  following  reasons: 

1.  These  periods  of  local  drops  or  rises  in  individual  reaction- 
time  curves  are  extremely  irregular  in  their  occurrence  and  do  not 
affect  all  the  strains  or  the  two  strains  of  the  same  line  equally. 

2.  There  is  no  coincidence  between  changes  in  reaction-time  and 
reproductive  index  and  no  demonstrable  relation  between  reaction- 
time  and  the  vigor  of  the  stock,  as  measured  by  the  reproductive 
index. 

3.  Cases  such  as  Lines  714,  794,  and  795,  in  which  these  non- 
genetic  differences  appeared  immediately  upon  the  beginning  of 
selection  (figures  7c,  lie,  and  12c),  can  scarcely  be  attributed  to 
depression  periods,  inasmuch  as  the  two  strains  of  each  of  these 
lines  had  just  been  derived  from  the  same  progenitor. 

4.  Males  or  ephippial  females  were  not  found  in  this  stock  at 
any  time  during  the  selection  experiments,  either  in  periods  of 
unusually  low  or  high  reproductive  indices  or  unusually  slight  or 
great  reactiveness;  yet  if  males  or  ephippial  females  occurred  in  any 
appreciable  numbers  (particularly  the  latter)  they  would  have  been 
detected. 

5.  Depression  periods  as  such  are  not  seen  in  the  stock  bred  in 
the  laboratory. 

The  writer  believes  that  these  fluctuations  are  merely  reflections 
of  relatively  local  environmental  influences  upon  the  individual 
strains  concerned. 

6.  ENVIRONMENTAL  CONDITIONS  AS  AFFECTING  VIGOR  OF  STOCK. 

Obviously  vigor  is  closely  related  to  environmental  factors.  As 
to  the  Cladocera  reared  in  this  laboratory,  there  is  reason  to  think 
that  their  vigor,  with  most  of  the  species  at  any  rate,  is  completely 
controlled  by  environmental  influences  and  is  not  affected  by  internal 
factors,  such  as  the  effect  of  long-continued  parthenogenetic  repro- 
duction. 

RELIABILITY  OP  THE  REPRODUCTIVE  INDEX. 

The  reproductive  index  is  a  safe  criterion  to  use  as  a  measure  of 
general  vigor  and,  it  is  believed,  of  general  muscular  activity  as  well. 
During  times  of  lowered  reproductive  activity  (appearing  in  decreased 
numbers  in  first  broods  and  in  increased  age  of  mothers  when  first 
broods  are  produced),  such  as  frequently  affected  several  or  all  of  our 
strains  of  the  same  species  at  the  same  time,  there  was  high  mortality, 
more  sterility,  more  cases  of  failure  to  produce  a  second  brood,  and 
frequently,  with  the  Daphnia  species,  a  noticeable  decrease  in  general 
muscular  activity  so  far  as  swimming  movements  were  concerned.1 

1  It  seems  surprising  that  this  is  not  reflected  in  increased  reaction-time  means,  but  (as  is 
shown  elsewhere)  there  is  extremely  little  evidence  of  a  relation  between  vigor  and  reaction-time. 
During  part  of  the  time,  when  the  stock  was  low  in  reproductive  vigor,  selections  were  omitted. 


144  SELECTION   IN    CLADOCERA    ON   THE    BASIS    OF 

Conversely,  when  the  reproductive  index  is  high,  mortality  is  low, 
sterility  is  extremely  rare,  there  are  few  failures  to  produce  a  second 
brood,  and  swimming  activity  is  at  its  maximum. 

COINCIDENT  FLUCTUATIONS  IN  VIGOR. 

If  vigor  is  related  to  environmental  conditions,  coincident  fluc- 
tuations in  reproductive  indices  for  the  two  strains  of  the  same  line 
and  for  the  strains  of  different  lines  should  be  in  evidence.  They  are 
present  and  most  conspicuous  when  the  two  strains  of  the  same  line 
are  compared. 

In  the  data  for  Line  757  (figure  17c)  for  example,  the  reproduc- 
tive indices  for  the  two  strains,  compared  in  each  case  with  the  repro- 
ductive index  for  the  preceding  period,  rise  or  decline  together  16 
times  and  move  in  reverse  directions  only  10  times.  Of  these  16 
coincident  fluctuations,  13  are  large  enough  to  have  a  probable  real 
meaning,  while  only  4  of  the  10  independent  fluctuations  are  large 
enough  to  be  considered  significant. 

There  is  not  more  coincident  fluctuation  in  reproductive  index 
in  Line  757  than  with  most  of  the  other  lines  (see  figures  Ic,  3 A,  7 A, 
SA,  HA,  12A,  13A,  and  17c),  so  that  in  general  there  is  a  rather  close 
correlation  between  the  reproductive  indices  of  the  two  strains  of  a 
line.  This  was  clearly  seen  to  be  a  reflection  of  environmental  in- 
fluences in  some  cases,  e.  g.,  the  August-November  1912  period 
(see  page  135),  and  it  is  believed  to  have  been  equally  due  to  environ- 
mental influences  where  the  precise  character  of  the  environmental 
influences  was  not  recognized. 

Coincident  fluctuations  in  reproductive  indices  are  likewise  seen 
between  the  different  lines  of  the  same  species  and  also  between  lines 
of  D.  pulex  and  S.  exspinosus. 

With  the  D.  pulex  lines  there  is  a  remarkable  agreement  in  the 
occurrence  of  low  reproductive  indices  for  the  August-November 
1912  period  (figures  Ic,  3 A,  7A,  and  8 A).  Line  740  (S.  exspinosus) 
likewise  shows  extremely  low  points  for  this  period  (figure  14A). 
There  are  some  early  data  for  Line  757  (figure  17c),  another  line  of  S. 
exspinosus,  culture  of  which  was  begun  during  the  month  of  Novem- 
ber 1912,  which  show  very  low  reproductive  indices.  This  is  a  striking 
fact,  as  showing  that  whether  the  lines  had  been  in  the  laboratory  for 
nearly  a  year  (as  in  the  case  of  the  Daphnia  pulex  lines),  for  4  months 
(Line  740),  or  had  just  been  introduced  into  the  laboratory  (Line 
757),  low  reproductive  indices  occurred.  As  has  been  stated  before 
(page  136),  this  was  a  period  of  particularly  poor  food  conditions  and 
it  involved  the  loss  of  much  laboratory  stock. 

During  June-July  1913,  7  of  8  strains  of  D.  pulex  (figures  Ic,  3A, 
TA,  and  SA)  show  lowered  reproductive  indices.  This  was  again  a 
period  of  not  very  successful  feeding  and  of  considerable  loss  of 


A   PHYSIOLOGICAL   CHARACTER.  145 

strains  of  the  D.  pulex  stock,  but  it  scarcely  affected  the  two  S. 
exspinosus  strains  (figures  14 A  and  ITc).1 

During  August-September  1913  all  the  D.  pulex  strains  and  all 
the  S.  exspinosus  strains  show  relatively  high  reproductive  indices. 
Again,  for  October-November  1914  all  the  strains  of  D.  pulex  and 
.5  (of  4)  strains  of  S.  exspinosus  have  high  reproductive  indices. 
These  are  particularly  marked  for  the  D.  pulex  strains.  For  the 
period  immediately  following,  December-January  1915,  most  of  the 
strains  of  both  species  show  considerably  lowered  though  not  particu- 
larly small  reproductive  indices  (figures  Ic,  7 A,  SA,  HA,  12A,  13A,  14A, 
and  17c).  In  October-November  1915  all  the  4  strains  of  &  exspino- 
sus and  3  of  the  4  strains  of  D.  pulex  have  high  reproductive  indices. 
Still  other  periods  of  more  or  less  coincident  fluctuations  in  repro- 
ductive indices  may  be  seen  in  the  curves. 

Mean  reproductive  indices  for  all  the  D.  pulex  strains  and  for  all 
the  S.  exspinosus  strains  were  determined,  and  it  was  found  that  the 
coincident  high  and  low  points  in  reproductive  index  as  given  above 
were  clearly  reflected  in  these  means. 

These  fluctuations  in  reproductive  indices  are  in  two  cases 
(August-November  1912  and  June- July  1913)  clearly  related  to 
poor  food,  and  perhaps  to  other  unfavorable  conditions,  and  it  is 
believed  the  other  coincident  fluctuations  are  likewise  associated  with 
environmental  factors. 

INDEPENDENT  FLUCTUATIONS  IN  VIGOR. 

Vigor  is  closely  related  to  environmental  conditions  and  many 
coincident  fluctuations  in  vigor  have  just  been  referred  to;  but  be- 
cause of  the  necessarily  somewhat  differential  treatment  (see  page  140) 
of  the  two  strains  of  the  same  line  (and  of  the  different  lines),  inde- 
pendent fluctuations  are  much  in  evidence  in  the  figures  showing 
reproductive  index  (figures  Ic,  SA,  TA,  SA,  HA,  12A,  13A,  14A,  and 
17c). 

For  example,  in  figure  17c,  showing  reproductive  indices  for  the 
two  strains  of  Line  757,  conspicuous  instances  in  which  the  changes 
in  reproductive  index  for  the  two  strains  of  this  line  are  in  reverse 
directions  (as  compared  with  the  preceding  period)  are  seen  in  the 
February-March  1914,  August-September  1916,  October-Novem- 
ber 1916,  and  February-March  1917  periods.  Six  other  less  con- 
spicuous fluctuations  (in  reverse  directions)  are  seen  in  this  same 
figure.  Some  of  these  small  fluctuations  are  probably  due  to  the 
reproductive  indices  for  the  two-month  period,  having  been  derived 
from  a  relatively  small  amount  of  data;  but  this  is  not  believed  to 
be  the  case  in  general  and  can  scarcely  be  assumed  to  account  for 

1  It  has  been  found  in  other  observations  that  while  the  physiology  of  these  two  species  of 
Cladocera  is  fundamentally  very  similar,  yet  physiological  differences  not  infrequently  appear. 
The  lack  of  a  pronounced  lowering  of  the  reproductive  indices  for  the  two  S.  exspinosus  lines  for 
this  period  is  an  illustration  of  these  physiological  differences  between  the  two  species. 


146  SELECTION    IN    CLADOCERA    ON    THE    BASIS    OF 

the  wide  fluctuations  in  the  four  periods  enumerated  above;  cer- 
tainly it  is  not  true  for  consistent  differences  in  reproductive  index 
between  the  two  strains  of  a  species  for  several  months  in  succession, 
such  as  are  seen  in  four  different  periods  for  Line  757  (figures  17c 
and  ISA)  and  in  parts  of  the  data  for  nearly  every  line  for  which 
reproductive  indices  are  given. 

Such  independent  fluctuations  are  believed  to  be  due  to  the 
somewhat  differential  treatment  resulting  from  the  transfer  of  young 
of  the  two  strains  of  a  line  on  different  days  and  into  culture-water 
of  different  collections.  The  cumulative  effect  of  continued  trans- 
fers into  somewhat  different  culture  media  seems  very  readily  to 
account  for  local  differences  in  reproductive  vigor  between  the  two 
strains  of  a  line. 

Some  of  these  independent  fluctuations  are  capable  of  explana- 
tion as  due  to  variations  in  an  internal  cycle  which  most  workers 
believe  exists  in  Cladocera.  But  differences  in  environmental  con- 
ditions explain  these  cases  so  readily,  some  of  the  contemporaneous 
changes  are  associated  with  recognized  changes  in  environmental 
conditions,  and  it  is  well  known  that  such  culture  media  do  have 
great  variation,  so  that  any  assumption  of  an  internal  cycle  seems 
quite  unnecessary.  Further,  in  Line  689  (figure  SA  and  B)  the  plus 
strain,  which  had  much  the  higher  reproductive  index  for  the 
first  two  months  of  selection,  had  a  considerably  lower  reproductive 
index  for  the  following  2  months.  This  would  seem  a  short  time  for 
the  internal  cycles  of  these  two  sister  strains  to  have  diverged.  The 
minus  strain  of  Line  714  (figure  7,  A  and  B)  had  a  reproductive  index 
nearly  three  times  as  large  as  that  for  the  plus  strain  for  the  first 
two-month  period.  Can  these  strains  just  derived  from  the  same 
individual  have  immediately  passed  into  different  periods  of  an 
internal  cycle?  There  are  other  cases  (less  striking,  however)  of 
significantly  different  reproductive  indices  immediately  or  very  soon 
after  the  origin  of  related  strains. 

However,  in  spite  of  the  unavoidably  somewhat  dissimilar  treat- 
ment of  the  two  strains  of  a  line  and  the  consequent  local  differential 
changes  in  reproductive  indices,  there  is  in  general  a  very  close 
correlation  between  the  reproductive  indices  of  kindred  strains,  a 
result  to  be  ascribed,  in  large  part  at  least,  to  the  general  influence  of 
similar  environmental  conditions. 

DIFFERENCES  BETWEEN  REPRODUCTIVE  INDICES  DURING  DIFFERENT  PARTS  OF  EXPERIMENT. 

The  reproductive  indices  for  the  two  strains  of  the  different 
lines  in  general  differed  somewhat  less  at  the  beginning  of  selection 
(for  the  first  4  months),  when  the  two  strains  had  just  been  separated, 
than  during  the  later  parts  of  the  experiment,  after  the  two  strains 
had  been  separate  for  considerable  periods  of  time  (figures  Ic,  SA, 
?A,  SA,  HA,  12A,  13A,  14A,  and  17c).  This  is  explainable  as  due  to 


A   PHYSIOLOGICAL   CHARACTER.  147 

the  fact  of  the  identity  of  environmental  conditions  before  selection 
was  begun  and  the  diversity  of  environmental  conditions  as  affecting 
the  two  strains  later  (or  as  due  to  their  being  in  different  portions  of 
their  cycles  at  the  same  time  later).  However,  some  of  the  strains, 
e.  g.,  Lines  695  and  689,  on  the  whole  show  greater  differences  in 
reproductive  indices  during  the  first  4  months  than  later  (see  figures 
Ic  and  3 A). 

Is  THE  INCREASED  REACTIVENESS  FOR  LINES  OF  S.  EXSPINOSUS  ASSOCIATED  WITH 

INCREASED  VIGOR? 

In  view  of  the  increase  in  reactiveness  in  the  S.  exspinosus  lines 
during  the  course  of  the  experiments,  it  is  of  interest  to  examine  the 
reproductive  indices  to  see  if  similar  or  other  changes  are  seen  in 
them. 

With  the  4  lines  of  Daphnia  pulex  (Lines  689,  695,  714,  and  719) 
for  which  reproductive  indices  are  worked  out,  there  are  in  general 
reproductive  indices  somewhat  below  the  average  for  the  first  2 
two-month  periods,  very  low  reproductive  indices  for  the  following 
2  periods  (during  the  obviously  poor  food  conditions),  and  there- 
after the  reproductive  indices  are  somewhat  fluctuating,  but  in 
general  fall  about  slightly  higher  means  throughout  the  remainder 
of  the  experiment  (figures  Ic,  3 A,  7A,  and  8 A).  Hence  with  the  lines 
of  D.  pulex  the  reproductive  index  increased  somewhat  during  the 
course  of  the  experiment. 

The  reproductive  indices  for  Line  740  show  marked  increases 
for  the  two-month  periods  just  following  the  first  4  months  of  selec- 
tion. These  early  low  reproductive  indices  are  clearly  related  to 
the  poor  food  conditions  prevalent  during  the  early  months  of 
selection  with  this  line  and  reflected  even  more  strongly  in  the  D. 
pulex  lines  which  had  been  in  the  laboratory  for  several  months 
previously.  There  is  also  a  rise  in  reproductive  index  for  Line  757 
after  the  first  2  months,  and  this  rise  is  only  partly  coincident  with 
recognized  differences  in  food  conditions  in  the  laboratory.  This 
rise  is  capable  of  explanation  as  due  to  the  cumulative  effect  of 
better  food  conditions  in  the  laboratory  than  in  the  outdoor  ponds, 
but  practically  the  maximum  rise  in  reproductive  index  for  this  line 
was  realized  at  once  (figure  17 c)1  and  there  is  no  evidence  for  a 
cumulative  effect  of  better  food  conditions.  The  reproductive 
indices  for  Line  795  are  much  higher  during  the  latter  part  of  the 
experiment  than  earlier  (figure  12A),  but  the  reverse  is  true  for  Line 
796  (figure  13 A).  On  the  whole,  the  reproductive  indices  for  S. 
exspinosus  do  not  show  any  general  increase  or  decrease  during  the 
course  of  the  experiment. 

Thus  it  is  seen  that  reproductive  indices  for  the  D.  pulex  strains 
increase  somewhat  during  the  course  of  the  experiment,  while  re- 

lrThe  general  level  for  the  plus  strain  was  not  higher  for  the  remainder  of  the  experiment, 
though  the  general  level  for  the  minus  strain  increased  somewhat  further. 


148  SELECTION   IN    CLADOCERA    ON   THE    BASIS    OF 

action-time  means  are  neither  increased  nor  decreased.  On  the  other 
hand,  reaction-time  means  are  decreased  for  S.  exspinosus  lines, 
while  there  are  no  consistent  general  changes  in  reproductive  indices. 

Perhaps  no  better  evidence  bearing  on  the  relation  between 
reproductive  vigor  and  reaction-time  can  be  had  (or  desired)  than 
that  obtained  from  the  following  data:  The  total  numbers  of  indi- 
viduals tested  in  making  the  selections  in  all  the  plus  strains  of  D. 
pulex  for  the  entire  series  of  the  experiments  was  6,991;  the  mean 
reaction-time  of  these  6,991  individuals  was  343.76  seconds.  The 
corresponding  figures  for  all  the  minus  strains  of  D.  pulex  were  6,973 
individuals,  with  a  mean  reaction-time  of  345.92  seconds.  The 
difference  was  only  2.16  seconds.  For  the  S.  exspinosus  plus  strains 
(except  Line  757  plus),  the  total  number  of  individuals  was  4,722 
and  the  mean  reaction-time  531.56  seconds.  For  the  S.  exspinosus 
minus  strains  (except  757  minus)  the  figures  were  3,943  individuals 
and  532.55  seconds.  The  difference  is  0.99  second. 

Since  in  both  species  the  plus  strains  in  general  were  somewhat 
more  vigorous  than  the  corresponding  minus  strains,  these  negligible 
differences  in  reaction-time  means  for  the  entire  series  of  experiments 
indicate  a  lack  of  relation  between  reproductive  index  and  reaction- 
time. 

POSSIBLE  DIVERGENCE  IN  VIGOR  BETWEEN  THE  Two"  STRAINS  OF  A  SELECTED  LINE. 

The  effect  of  selection  on  the  basis  of  reaction  to  light  might 
conceivably  result  in  plus  strains  of  greater  vigor  and  minus  strains 
of  reduced  vigor.  It  would  indeed  seem  possible  that  the  minus 
strains  might  become  very  much  reduced  in  vitality  and  perhaps 
eventually  die  out  if  selection  were  continued  long  enough.  In  view 
of  this  theoretical  possibility  it  is  interesting  to  note  that  of  the 
selection  experiments  discontinued  because  of  loss  of  one  or  both  of 
the  strains  (see  table  1,  column  8),  2  lines  were  discontinued  because 
of  the  loss  of  both  strains  within  a  few  days  of  each  other,  1  line 
suffered  the  loss  of  the  minus  strain  only,  and  5  lines  suffered  the 
loss  of  the  plus  strain  only.1  Hence  this  significant  bit  of  data  does 
not  indicate  reduced  vitality  in  the  minus  strains  as  compared  with 
the  corresponding  plus  strains. 

Column  9  of  table  1  likewise  throws  light  upon  a  point  inti- 
mately associated  with  the  general  vitality  of  corresponding  plus  and 
minus  strains,  the  relative  numbers  of  generations  of  descent  of  the 
plus^and  corresponding  minus  strains.  This  column  shows  the  total 
number  of  generations  of  descent  of  the  different  strains  at  the  close 
of  selection  within  the  different  lines.  In  8  lines  the  plus  strain  had 
descended  for  a  greater  number  of  generations  than  the  corresponding 
minus  strains;  in  7  the  minus  had  descended  farther;  and  in  1  line 

1  Two  of  the  last  were  lost  through  accident,  however,  rather  than  because  of  general  weak- 
ness or  poor  food  conditions,  and  one  of  the  minus  strains  suffered  the  accidental  loss  of  part 
of  its  stock,  except  for  which  it  might  have  survived. 


A   PHYSIOLOGICAL   CHARACTER.  149 

the  2  strains  had  descended  an  equal  number  of  generations.  The 
plus  strains  which  had  descended  a  greater  number  of  generations 
averaged  an  advance  of  2.9  generations  ahead  of  their  respective 
minus  strains;  the  minus  strains  which  had  descended  farther  than 
their  corresponding  plus  strains  averaged  an  advance  of  4.3  genera- 
tions beyond  the  plus  strains.  The  D.  pulex  and  S.  exspinosus  lines 
were  pretty  well  divided  as  to  whether  the  plus  or  minus  strains  had 
descended  the  greater  number  of  generations.  In  all  the  D.  longispina 
lines,  however,  the  plus  strain  had  descended  farther — by  a  single 
generation  in  two  of  the  lines,  but  by  5  generations  (almost  11  per 
cent)  in  one.  Hence,  there  was  possibly  some  real  loss  of  vigor  in 
the  minus  strains  of  D.  longispina  as  compared  with  'their  respective 
plus  strains  (though  three  lines  are  too  small  a  number  from  which  to 
reach  a  safe  conclusion).  But  with  S.  exspinosus  and  D.  pulex  there 
was  no  loss  in  vigor  of  the  minus  as  compared  with  their  respective 
plus  strains,  so  far  as  was  revealed  by  their  total  numbers  of  general 
tions  of  descent.  In  fact  (so  far  as  this  point  is  concerned),  the  minus 
strains  in  5  of  the  8  D.  pulex  lines  descended  farther  and  by  larger 
margins  (an  average  of  5.2  generations  as  compared  with  3.7  genera- 
tions for  the  plus  strains),  which  affords  almost  as  much  ground  for 
assuming  that  the  plus  strains  of  the  D.  pulex  lines  became  of  inferior 
vigor  compared  with  the  corresponding  minus  strains  as  is  afforded  for 
the  opposite  conclusion  with  reference  to  the  D.  longispina  lines. 

Some  effect  of  the  selections  upon  the  general  vigor  of  the 
corresponding  plus  and  minus  strains  is  evident,  however,  in  some 
cases  in  the  generally  lower  reproductive  indices  for  some  of  the 
minus  strains  as  compared  with  their  corresponding  plus  strains.  In 
only  1  (Line  740)  of  the  8  lines  for  which  the  reproductive  indices 
have  been  worked  out  was  the  minus  strain  in  general  conspicuously 
the  more  vigorous  of  the  2  strains  of  the  same  line  (figure  MB).  It 
is  significant,  however,  that  (except  possibly  for  parts  of  the  data 
for  Lines  795  and  796,  and  these  differences  were  not  maintained) 
the  differences  in  vigor  of  the  two  strains  of  the  same  line  were  not 
cumulative;  and  in  no  case  were  they  greater  during  the  later  than 
during  the  early  portions  of  the  experiments,  except  for  the  very 
first  months  of  selection.  It  would  seem,  then,  that  in  some  cases 
there  was  a  tendency  for  the  selected  individual  (the  most  reactive 
individual)  in  the  plus  strain  to  be  more  vigorous  than  the  selected 
individual  (the  least  reactive  individual)  in  the  corresponding  minus 
strains,  but  that  this  difference  is  neither  universal,  uniform,  nor 
cumulative  in  its  effect.  As  is  shown  elsewhere,  differences  in  re- 
productive indices,  even  when  relatively  large,  are  not  associated  with 
corresponding  differences  in  reaction-time.1 

xThe  lack  of  relation  between  reproductive  index  and  reaction-time  has  been  referred  to 
in  several  places  (see  pages  40-45,  49,  60,  65,  72,  76,  80,  83,  88-89,  and  108-115)  and  need 
not  be  further  discussed  here. 


150       SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

STATEMENT  OF  RESULTS  OF  SELECTION. 

The  results  of  this  series  of  experiments  may  be  stated  briefly: 

With  two  lines,  Lines  689  and  719,  there  were  found  reaction- 
time  differences  in  the  reverse  of  selection.  These  differences  were 
so  consistent  as  to  suggest  real  genetic  differences  affecting  reactive- 
ness  in  the  two  strains  of  these  two  lines,  though  Line  689  was  carried 
for  a  relatively  short  period  of  time.  In  the  case  of  Line  719  the 
reaction-times  differed  less  toward  the  end  of  the  experiment,  and  it 
is  suggested  that  selection  may  have  operated  to  produce  this  effect. 

The  results  with  Line  689  and  Line  719  suggest  that  in  addition 
to  genetic  differences  in  the  direction  sought  in  selection  genetic 
changes  may  occur  in  the  opposite  direction.  This  is  strongly  sug- 
gestive of  mutation.  It  does  not,  however,  obviate  the  theoretical 
difficulty  of  explaining  by  mutation  the  plural  genetic  changes  in 
Line  757. 

Lines  695,  713,  714,  751,  766,  768,  and  796  show  no  evidence 
(or  slight  evidence  as  in  Line  713)  of  significant  differences  in  reactive- 
ness  between  their  plus  and  minus  strains.  Some  of  these  lines  were 
carried  for  relatively  short  periods  of  time,  but  Lines  695,  713,  714, 
and  perhaps  768  and  796  were  subjected  to  selection  for  what  would 
seem  adequate  periods  of  time. 

Line  691  shows  a  possible  effect  of  selection,  though  such  an 
effect  is  not  fully  supported  by  the  last  year's  data  and  may  therefore 
be  disregarded.  Selection  was  conducted  for  27  months,  84  and  85 
generations  in  the  two  strains. 

Line  711  apparently  shows  an  effect  of  selection,  but  the  differ- 
ences are  small  and  the  evidence  is  not  entirely  conclusive,  though 
the  case  is  as  convincing  as  the  result  in  the  opposite  direction  for 
Lines  689  and  719.  Selection  was  continued  for  45  and  46  genera- 
tions, covering  a  period  of  about  16  months. 

Line  762  gave  a  possible  effect  of  selection  during  the  last  two 
months  of  the  experiment.  In  view  of  the  wide  fluctuations  in  the 
reaction-time  curves  of  many  of  the  lines,  this  is  not  sufficient  data 
upon  which  to  base  a  conclusion,  but  the  entire  consistency  in  the 
differences  between  broods  of  the  plus  and  minus  strains  during  this 
late  portion  of  the  experiment  is  unusual  enough  to  be  highly  sug- 
gestive. However,  in  view  of  the  local  differences  in  reactiveness 
seen  in  other  lines  (see  pages  140-142),  an  effect  of  selection  in  Line 
762  must  be  considered  problematical.  The  plus  strain  was  lost, 
thereby  terminating  the  experiment  after  36  and  35  generations  of 
selection. 

Line  794  may  have  undergone  a  divergence  in  reactiveness  during 
selection.  For  the  first  6  months  the  differences  were  in  the  reverse 
of  selection,  but  the  remaining  12  two-month  periods  gave  differences 
in  the  opposite  direction,  with  3  exceptions,  and  8  of  these  differences 


A   PHYSIOLOGICAL   CHARACTER.  151 

were  relatively  large.  The  single  test  series,  however,  quite  fails  to 
lend  support  to  a  difference  in  reactiveness  between  the  two  strains, 
and  this  case  likewise  remains  doubtful. 

Line  795  clearly  shows  a  lack  of  selective  effect  during  most  of 
the  experiment.  The  last  month's  data  is  very  suggestive  of  signi- 
ficant reaction-time  differences,  but  the  data  are  too  slight  in  amount 
to  receive  serious  consideration. 

A  mutation  possibly  occurred  in  the  minus  strain  of  Line  740, 
causing  it  to  be  more  reactive  to  light.  The  case  was  critically 
examined  (page  89)  and  the  conclusion  was  reached  that  the  unusual 
reactiveness  of  a  few  broods  of  this  strain  was  probably  due  to 
peculiar  local  environmental  conditions. 

None  of  the  above  possible  cases  of  differences  in  reactiveness 
between  the  two  strains  of  a  line  is  well  enough  supported  by  the 
evidence  to  be  considered  proven.  The  cases  best  supported  by  the 
evidence  are  in  Lines  689,  719,  and  711.  In  Lines  689  and  719  the 
differences  are  in  the  reverse  of  selection.  With  Line  719  the 
difference  was  slight  during  the  latter  portion  of  the  experiment. 
Line  689  was  lost  before  enough  evidence  was  obtained  to  convince 
one  that  the  difference  was  necessarily  genetic.  Line  711  shows  con- 
sistent differences  in  accordance  with  an  effect  of  selection,  but,  like 
Line  689,  was  lost  before  enough  evidence  was  obtained  to  convince 
one  that  the  difference  was  certainly  genetic. 

With  Line  757,  on  the  other  hand,  a  marked  effect  of  selection  is 
attested  by  every  significant  bit  of  evidence  which  the  data  for  this 
line  provide.  The  divergence  in  mean  reaction-times  began  to  appear 
after  the  first  3  months  at  the  beginning  of  the  experiment  (for  which 
the  two  strains  had  the  same  reaction-time  means).  For  the  second 
(two-month)  period  the  means  differed  by  —25  ±22.82  seconds,  and 
the  means  for  the  following  two-month  period  differed  by  —72  ±17.78 
seconds,  a  difference  of  statistical  significance  (4.05  times  the 
probable  error),  in  spite  of  the  small  numbers  and  consequently  large 
probable  error.  This  is  followed  by  6  two-month  periods,  during 
^  which  the  differences  are  all  in  the  same  direction  and  3  of  which 
(even  for  the  shorter  periods)  are  of  large  statistical  significance, 
though  the  differences  of  —55,  —63,  and  —70  seconds  were  not  of 
statistical  value  because  of  large  probable  errors.  For  the  two-month 
period  June- July  1914  the  minus  strain  was  the  more  reactive  and 
the  difference  was  132  ±29.79  seconds,  a  difference  4.4  times  the 
probable  error.  For  the  following  two-month  period  the  difference 
is  only  -2  seconds.  From  this  period  on,  all  the  16  differences  (by 
two-month  periods)  indicate  greater  reactiveness  in  the  plus  strain. 
With  3  exceptions  the  differences  were  large  and  of  statistical  value, 
even  when  considered  by  two-month  periods.  One  of  these  smaller 
differences,  which  was  not  of  statistical  value,  occurred  within  3 


152  SELECTION    IN    CLADOCERA    ON   THE    BASIS   OF 

months  of  the  close  of  the  experiment,  when  the  effect  of  selection 
was  at  its  height.1 

Considering  the  data  by  longer  periods,  those  from  the  beginning 
of  the  experiment  to  July  1914  show  significant  differences  in  mean 
reaction-time,  though  the  early  differences  were  generally  not  large. 
Following  this  was  a  period  during  which  both  strains,  and  particu- 
larly the  minus  strain,  fluctuated  widely,  though  the  difference  for 
this  longer  period  is  significant  of  a  real  difference  in  reactiveness 
between  the  two  strains.  During  the  latter  part  of  1915  the  dis- 
turbances due  to  environmental  factors  became  less  marked  and 
the  divergence  in  mean  reaction-time  appeared  to  increase  greatly 
and,  with  a  single  fluctuation,  continued  very  large  during  the 
remainder  of  the  experiment.  During  the  final  9  months  of  the 
experiment  the  mean  for  the  plus  strain  was  only  one-third  that  for 
the  minus  strain.  The  form  of  the  curves  (figure  18s)  suggests  that 
the  divergence  in  reactiveness  was  still  increasing  at  the  close  of  the 
experiment. 

Points  of  importance  in  connection  with  the  results  with  Line 
757  are  that  both  the  plus  and  minus  strains  became  modified  during 
selection  and  that  the  divergence  was  permanent  or  at  least  persisted 
for  112  generations.  It  is  also  important  to  note  that  there  is  nothing 
to  distinguish  the  two  strains  other  than  the  difference  in  behavior 
of  the  strains  in  the  experimental  tank  in  response  to  directive 
stimulation  by  light,  and  a  somewhat  higher  reproductive  index  for 
the  plus  strain  during  most  but  not  during  the  concluding  months  of 
the  experiment. 

DISCUSSION  OF  RESULTS. 

Consistent  differences  in  reactions  to  light  of  animals  of  the  same 
or  closely  related  stocks  appear  not  to  have  been  frequently  found. 

McEwan  (1918)  presents  a  conclusive  case  of  differences  in 
phototropism  which  are  clearly  genetic.  A  certain  mutant  strain, 
"tan,"  of  Drosophila  melanogaster,  unlike  most  of  this  species, 
showed  very  little  response  to  light.  Tan  is  a  very  variable  character 
and  can  not  always  be  distinguished  by  inspection  of  the  flies.  The 
differences  in  phototropism  between  tan  and  normal  flies  were  so 
striking  that  it  was  found  (and  proven  by  breeding  tests)  that  the 
slight  phototropism  of  tan  stock  was  a  more  reliable  means  of  diag- 
nosing tan  than  inspection  of  the  flies.  The  character  was  shown  to 
be  a  recessive  sex-linked  character.  McEwan  found  differences  in 

1Such  a  marked  reduction  in  the  difference  between  the  means,  coming  at  a  time  when 
according  to  all  criteria  the  two  strains  were  so  different  in  their  reactiveness  to  light,  seems 
certainly  due  to  unequal  local  environmental  influences,  which  may  also  account  for  the  relatively 
wide  fluctuations  in  the  reaction-time  curves  for  the  period  from  June  1914  to  July  1915  (see 
pages  141  and  159-160).  While  during  this  earlier  period  the  minus  strain  for  1  two-month 
period  had  the  lower  reaction-time  mean,  this  fact  need  perhaps  excite  no  more  surprise  than 
the  fact  that  the  mean  for  the  minus  strain  was  so  near  that  for  the  plus  strain  at  the  later  period 
when  the  general  reactiveness  of  the  two  strains  was  so  widely  different. 


A   PHYSIOLOGICAL   CHARACTER.  153 

phototropism  of  other  types  of  Drosophila,  but  these  were  associated 
with  differences  in  the  color  or  other  conditions  of  the  eye  itself.  In 
the  tan  mutant,  on  the  other  hand,  no  peculiarities  could  be  dis- 
covered in  the  eye. 

The  writer  has  considerable  (unpublished)  data  tending  to  show 
that  the  amphipod  Eucrangonyx  gracilis,  living  in  the  surface  streams 
near  Bloomington,  Indiana,  is  less  negatively  phototropic  than  the 
same  species  which  lives  in  Mayfield's  Cave,  near  at  hand.  The 
differences  in  reaction  are  not  large  and  consistent  enough  to  be 
very  conclusive.  In  this  species  those  inhabiting  caves  and  those 
inhabiting  the  open  are  indistinguishable  morphologically,  except  for 
differences  in  pigmentation,  the  former  having  fully  pigmented  eyes, 
but  no  body  pigment. 

In  asexually  reproducing  forms1  Jennings  (1908,  1909)  with 
Paramecium,  Agar  (1913,  1914)  with  Cladocera,  Ewing  (1914)  with 
aphids,  Hanel  (1908)  and  Lashley  (1915,  1916)  with  Hydra,  and 
other  workers  have  failed  to  obtain  hereditarily  diverse  strains  by 
selection  within  the  uniparental  progeny  of  single  individuals. 

On  the  other  hand,  Middleton  (1915)  in  Stylonychia,  Stout  (1915) 
with  Coleus,  Jennings  (1916)  with  Difflugia,  Root  (1918)  in  Centro- 
pyxis,  and  Hegner  (1918,  1919)  with  Arcella  seem  to  have  found 
genetic  variation  wherever  they  sought  it. 

Stocking  (1915),  studying  abnormalities  in  Paramecium,  found 
that  of  122  strains  containing  abnormal  individuals,  in  which  the 
character  was  clearly  hereditary,  97  were  persistently  abnormal 
races,  i.  e.,  they  presumably  were  not  subject  to  genetic  variation 
in  this  regard;  and  25  strains  were  subject  to  genetic  variation,  i.  e., 
selection  was  effective  in  25  strains.  That  is  to  say,  about  20  per  cent 
of  the  strains  studied  by  her,  in  which  the  character  was  inherited, 
were  subject  to  genetic  variation. 

The  results  of  selection  with  Protozoa  are  open  to  certain  objec- 
tions which  Jennings  (1916,  p.  523)  outlines  somewhat  as  follows: 
(1)  that  the  results  may  be  due  to  cytoplasmic  influences  and  hence 
may  not  be  of  real  genetic  significance;  (2)  that  the  result  may  be 
due  to  nuclear  recombination  between  the  active  nucleus  and  nuclear 
material  within  the  cytoplasm  (chromidia);  and  (3)  that  nuclear 
recombination  may  occur  between  different  nuclear  masses  in  the 
same  individual.  Professor  Jennings  (1916,  p.  525)  believes  that 
these  possibilities  have  "no  claim  to  greater  probability  than  their 
negatives,  so  far  as  our  knowledge  of  the  facts  is  concerned. "  The 
opinion  of  different  workers  as  to  the  validity  of  these  objections 
differs  greatly. 

1  In  view  of  the  many  excellent  historical  accounts  which  have  appeared  it  does  not  seem 
desirable  to  review  any  considerable  portion  of  the  vast  amount  of  work  on  selection  within  the 
pure  line.  Excellent  discussions  of  this  literature  are  given  by  Jennings  (1910, 1917),  Pearl  (1917), 
and  Sturtevant  (1918). 


154       SELECTION  IN  CLADOCERA  ON  THE  BASIS  OF 

Cladocera  material  has  the  advantages  for  a  selection  experiment 
possessed  by  protozoa  in  that  it  is  (under  controlled  conditions) 
exclusively  uniparental  in  inheritance.  It  has  additional  advantages 
in  that  the  germ-plasm  is  less  bound  up  with  the  soma  than  in 
protozoa  and  the  very  great  advantage  that  (so  far  as  our  knowledge 
goes,  at  any  rate)  there  are  no  complications  of  chromidia  or  separate 
nuclear  masses  within  the  germ-cell,  recombination  of  which  might 
conceivably  produce  the  results  attained  by  selection  in  protozoa. 

So  far  as  known,  there  is  no  mechanism  within  the  germ-cells 
of  Cladocera  through  which  a  segregation  or  recombination  of  nuclear 
material  is  to  be  expected  in  parthenogenesis.  In  the  production 
of  the  parthenogenetic  egg  there  is  a  single  maturation  division 
(Weismann,  1886;  Kuhn,  1908),  in  which  the  chromosomes  behave  as 
in  any  ordinary  cell-division,  i.  e.,  each  chromosome  is  merely  divided 
into  two  theoretically  equal  parts,  equal  quantitatively  and  quali- 
tatively. Hence  the  objections  offered  to  results  of  selection  with 
protozoa  do  not  apply  to  the  present  material.  Any  results  obtained 
from  selection  in  such  material  (parthenogenetic  and  without  reduc- 
tion) should  have  a  crucial  bearing  on  the  problem  of  selection. 

In  experiments  having  to  do  with  the  variability  of  organisms 
it  does  not  seem  necessary  to  suppose  that  all  apparently  similar 
material  should  be  found  similarly  subject  to  genetic  variation. 
In  Stocking's  (1915)  Paramecium,  about  20  per  cent  of  the  strains 
in  which  an  abnormality  was  hereditary  showed  genetic  variation. 
In  the  Cladocera  material  used  in  selection,  Line  757  alone  has 
certainly  shown  such  genetic  variation  with  regard  to  the  character- 
istic studied,  though  the  possible  (but  doubtful)  mutation  in  the 
minus  strain  of  Line  740  and  the  somewhat  consistent  differences 
in  mean  reaction- times  between  the  two  strains  of  Lines  689,  691, 
711,  719,  762,  794,  and  795  are  suggestive  of  differences  of  genetic 
significance.  Allowing  that  the  case  is  proven  for  Line  757  alone 
(though  some  of  the  other  cases  are  very  suggestive),  genetic  variation 
was  found  in  only  one  of  the  15  distinct  lines  of  Cladocera  studied. 
This  is  approximately  only  7  per  cent. 

The  effects  of  selection  in  Line  757  may  be  assumed  to  be 
either  (1)  of  a  nature  of  general  physiological  changes,  possibly 
changes  in  metabolism  or  some  kindred  effect,  or  (2)  direct  genetic 
changes.1 

If  the  former  interpretation  is  favored,  the  question  arises  as 
to  what  is  the  basis  of  such  physiological  effects.  If  they  are  con- 
sidered not  strictly  genetic,  the  apparently  permanent  nature  of  the 
change  is  a  matter  of  especial  importance.  Cases  of  transmission 

1  In  this  discussion  genetic  variations,  in  metazoa  at  least,  are  assumed  to  concern  genetic 
factors  confined  to  the  chromosomes.  The  carrying  over  from  one  generation  to  another  of 
cytoplasmic  inclusions  and  the  transmission  of  variations  or  modifications  which  affect  only  one 
or  two  generations  are  assumed  to  concern  the  cytoplasm  alone  and  are  considered  not  strictly 
genetic. 


A   PHYSIOLOGICAL   CHARACTER.  155 

not  really  genetic  and  some  of  which  are  clearly  cytoplasmic  are  well 
known.  If  Sudan  III  is  fed  to  fowls  some  of  the  dye  is  deposited  in 
the  egg-yolk  (Riddle,  1908)  and  reappears  in  certain  of  the  tissues 
of  the  chick  (Gage  and  Gage,  1908).  This  is  merely  the  carrying 
over  of  a  cytoplasmic  inclusion.  Other  cytoplasmic  inclusions,  the 
plastids  of  plants,  parasitic  or  symbiotic  bacteria  and  protozoa,  are 
carried  over  in  the  cytoplasm  of  the  egg;  but  these  are  mere  inclusions 
and  this  is  not  true  inheritance.  Starving  occasionally  causes  the 
production  of  young  of  small  size.  The  effect  would  seem  probably 
due  to  reduced  quantity  of  food  material  (or  to  deleterious  sub- 
stances) in  the  egg  cytoplasm.  Later  generations  are  not  affected. 
The  effects  of  immunization  are  transmitted  by  the  mother  and  not 
by  the  father.  The  effects  are  not  permanently  transmitted  and 
are  presumably  cytoplasmic.  Agar  (1913)  was  able  to  produce 
young  Cladocera  with  carapace  gaping  open  by  keeping  the  mothers 
in  a  particular  food  medium.  This  peculiarity  was  transmitted  for 
one  generation  by  affected  stock  when  reared  in  the  usual  culture 
medium,  but  the  abnormality  rapidly  decreased  and  ceased  to  appear 
after  one  generation.  This  transmission  may  readily  have  been 
cytoplasmic,  but  it  is  clearly  not  a  case  of  real  inheritance.  The 
substances  carried  over  in  the  egg  cytoplasm  (if  there  were  such) 
were  rapidly  dissipated  and  lost. 

These  are  typical  cases  of  cytoplasmic  transmission,  and  one 
may  conclude  with  Conklin  (1920)  that -"in  the  present  state  of 
our  knowledge  there  is  not  sufficient  evidence  to  conclude  that 
modifications  of  the  cytoplasm  of  germ-cells  are  ever  really  inherited 
or  that  they  are  ever  the  initial  steps  in  evolution/' 

To  assume  that  the  changes  in  phototropism  in  the  present  case 
were  not  really  genetic  would  seem  to  place  them  in  the  class  of 
cytoplasmic  transmissions  and  to  assume  a  permanent  cytoplasmic 
or  extra-genetic  transmission  which  in  our  present  state  of  knowledge 
would  seem  unwarranted. 

Aside  from  its  theoretical  difficulty,  there  does  not  seem  to  be 
any  evidence  favoring  the  assumption  of  general  and  purely  physio- 
logical changes  as  accounting  for  the  divergence  in  reactiveness  of 
the  two  strains  of  Line  757,  since  these  two  strains  are  indistinguish- 
able, except  in  their  reactiveness  to  light. 

However,  it  is  of  course  possible  to  assume  that  a  non-chromo- 
somal inheritance  occurs  and  to  apply  this  interpretation  to  the 
present  results,  though  the  writer  can  see  no  reason  for  favoring  such 
an  interpretation.  If,  however,  this  should  prove  to  be  the  correct 
interpretation  a  practically  new  phase  of  inheritance  is  opened  up, 
and  in  any  case  the  importance  of  the  present  results  as  bearing 
upon  selection  within  the  pure  line  is  not  lessened. 

If,  on  the  other  hand,  physiological  changes  having  a  genetic 
basis  be  assumed  to  explain  the  present  results  of  selection,  the  nature 
of  the  factorial  changes  involved  is  still  the  pertinent  question. 


156  SELECTION    IN    CLADOCERA    ON    THE    BASIS    OF 

If  the  variations  are  considered  direct  genetic  changes  (or 
general  physiological  changes  having  a  genetic  basis),  they  may  be 
thought  of  as  having  been  due  to  segregation,1  to  larger  mutations, 
or  to  many  small  genetic  changes. 

It  would  seem  that  the  effect  of  selection  here  obtained  can  not 
be  explained  as  due  to  recombination  or  segregation  unless  segrega- 
tion occurred  rather  generally  in  Line  757  under  conditions  in  which 
it  is  not  known  and  is  theoretically  not  supposed  to  occur.  It  is 
conceivable,  however,  that  segregation  may  occur  on  rare  occasions 
in  Cladocera  in  the  germ-tract  previous  to  maturation,  in  the  one 
maturation  division  (even  though  that  is  a  non-reduction  division), 
or  in  cleavage  or  later  somatic  divisions.  But  this  explanation  for 
the  result  with  Line  757  encounters  certain  difficulties:  (1)  Such 
factorial  recombination,  if  large  quantitatively,  should  at  once  pro- 
duce an  obvious  effect.  (2)  If,  on  the  other  hand,  the  factorial  re- 
combination produces  only  a  slight  effect,  the  result  obtained  with 
Simocephalus  Line  757  is  explicable  only  as  the  result  of  several 
of  these  exceptional  factorial  changes.  (3)  The  effect  is  in  both  high 
and  low  selection  strains. 

Since  with  Line  757  the  effect  is  extensive,  is  in  both  strains,  and 
appears  gradually  and  not  by  a  sudden  increment  (except  possibly, 
but  doubtfully,  at  one  point  in  the  minus  strain),  this  sort  of  explana- 
tion would  call  for  more  than  a  single  factorial  recombination. 
Probably  many  more  genetic  changes  occurred,  but  at  least  three 
such  genetic  recombinations  are  required  to  explain  the  case  on  this 
basis.  Such  might  possibly  be  located  as  follows:  one  in  the  minus 
strain  early  in  its  laboratory  history,  one  in  the  plus  strain,  possibly 
about  January  1915,  and  one  in  the  minus  strain  about  August  1915 
(figure  18fi).  But  the  points  at  which  these  three  or  more  hypothet- 
ical recombinations  occurred  are  not  indicated  in  the  data  by  broods 
(tables  41  and  42)  and  can  be  doubtfully  and  only  approximately 
located  (assuming  that  they  were  masked  for  a  time  by  environ- 
mental conditions)  by  reference  to  the  averages  by  two-month  periods 
(tables  43  and  44  and  figure  18B).  Yet  presumably  these,  at  least 
the  third  one2  (assuming  a  minimum  of  three  genetic  modifications), 
must  have  been  genetic  changes  of  large  moment,  considering  the 
wide  differences  in  reactiveness  between  the  two  strains  during  the 
latter  portion  of  the  experiment.  Since  these  factorial  changes  must 
have  come  about  under  exceptional  conditions,  three  or  more  such 
occurrences  affecting  two  strains  of  the  same  line  within  a  limited 
period  seem  unlikely  of  attainment. 

NOTE. — There  is  another  possible  suggestion  concerning  segregation  in  this  material: 
that  notwithstanding  parthenogenesis  and  the  presumed  lack  of  segregation  in  maturation 
there  may  be  a  mechanism  in  Cladocera  by  which  changes  in  genetic  constitution  may, 

1  Including  the  phenomena  of  crossing-over  and  non-disjunction. 

*As  shown  on  page  159,  there  is  strong  probability  that  no  large  genetic  change  occurred 
at  this  point. 


A   PHYSIOLOGICAL   CHARACTER.  157 

in  certain  cases,  be  readily  brought  about  in  parthenogenetic  reproduction.  This  is  not 
indicated  by  the  cytological  evidence,  so  far  as  known,  and  would  seem  to  call  for  quite 
a  novel  type  of  cytological  behavior  for  which  there  is  no  direct  genetic  or  cytological 
evidence  in  parthenogenetic  organisms.  But  should  there  prove  to  be  such  a  segregation 
in  Simocephalus,  then  the  effects  of  selection  in  this  experiment  with  Cladocera  readily 
yield  to  explanation  in  conformity  with  the  chromosome  interpretation  of  the  pure-line 
hypothesis.  Otherwise  the  writer  does  not  see  how  the  results  can  be  satisfactorily  made  to 
conform  with  this  theory.  The  Cladocera  material  deserves  a  critical  cytological  analysis. 
The  writer  would  gladly  cooperate  in  providing  material  for  such  a  study  by  a  cytologist 
who  could  give  ample  time  to  the  problem. 

It  is  just  possible,  however,  that  there  may  be  an  unusual  or  at  any  rate  virtually 
unknown  type  of  chromosomal  behavior  in  Cladocera,  permitting  segregation  more  fre- 
quently than  would  otherwise  be  expected  in  material  in  which  segregation  seems  improb- 
able. Wenrich's  (1916)  observations  on  Phrynotettix  are  most  suggestive  in  this  connection. 
He  found  reduction  occurring  in  a  single  chromosome  and  apparently  not  affecting  the 
other  chromosomes  in  that  division.  Crossing-over  and  non-disjunction  (which  Bridges 
and  others  have  demonstrated  and  utilized  so  successfully  in  interpreting  genetic  results 
with  Drosophila  and  other  forms)  may  be  appealed  to  as  affording  possible  vehicles  for  such 
genetic  changes,  assuming  that  several  genetic  factors  are  involved  in  the  reactiveness  to 
light  in  Cladocera  and  that  Line  757  was  originally  heterozygous  for  certain  of  these  factors. 
In  view  of  such  known  irregularities  in  chromosome  behavior,  one  is  led  to  consider  the 
possibility  that  a  partial  segregation  may  occur  in  the  one  maturation  division  in  partheno- 
genetic reproduction  in  Cladocera.  Nabours  (1919)  found  segregation  in  parthenogenetic 
reproduction  in  one  of  the  grouse  locusts,  Apotettix.  However,  judging  from  the  very 
limited  cytological  evidence  presented,  it  would  seem  that  in  this  case  reduction  was  fully 
accomplished,  as  in  sexual  reproduction. 

However,  there  is  no  cytological  evidence  that  reduction  or  segregation  of  any  sort 
occurs  in  the  one  maturation  division  of  parthenogenetic  eggs  of  Cladocera  and,  even 
assuming  that  such  a  partial  segregation  may  occur  in  the  single  maturation  division  in 
Cladocera,  there  is  still  the  theoretical  difficulty  of  accounting  for  such  frequent  genetic 
change  as  seems  to  have  occurred  in  Line  757.  For  it  is  hard  to  see  how,  even  with  assumed 
partial  reduction  in  Cladocera  (which  lack  the  complicated  features  of  protozoan  nuclei), 
the  rate  of  genetic  change  should  so  greatly  have  exceeded  that  so  carefully  worked  out 
for  Drosophila.  This  is  not  necessarily  a  fatal  objection,  however.  Even  though  Cladocera 
are  probably  very  little  subject  to  genetic  change,  once  genetic  change  occurs,  for  any 
thing  we  know  to  the  contrary,  it  may  proceed  at  a  rapid  rate. 

This  possible  explanation  has  a  certain  appeal  to  the  writer,  not  only  because  it  offers 
an  explanation  (in  accord  with  the  explanation  offered  for  most  results  of  selection)  for 
the  results  attained  in  the  present  study  of  the  effects  of  selection  on  the  basis  of  reactive- 
ness  to  light,  but  also  because  certain  other  (unpublished)  results  of  study  of  Cladocera 
material  may  in  part  be  explained  on  this  basis. 

Explanation  of  the  result  with  Line  757  by  larger  mutations 
encounters  considerable  difficulty:  (1)  the  necessity  for  assuming 
at  least  three  mutations,  at  least  one  of  which  must  have  been  quite 
large;  (2)  the  lack  of  definite  points  at  which  mutations  may  be 
presumed  to  have  occurred;  and  (3)  the  occurrence  of  an  additional 
mutation  or  mutations  effective  in  the  same  direction  as  an  earlier 
one.  Further,  the  mutations  in  the  two  strains  must  have  occurred 
in  opposite  directions,  rendering  the  plus  strain  more  reactive  and 
the  minus  strain  less  reactive. 

In  general,  in  uniparental  inheritance  mutation  may  perhaps  be 
expected  to  be  only  half  as  frequent  as  in  biparental  inheritance, 
since  in  the  former  a  single  germ-cell  is  involved  in  the  production  of 
each  individual  offspring.  Bridges  (1919)  states  evidence  tending  to 
show  that  in  Drosophila  mutations  may  occur  in  oogonial  (or  sperma- 
togonial)  divisions  and  in  the  zygote  "  immediately  after  fertiliza- 


158  SELECTION    IN    CLADOCERA    ON   THE    BASIS   OF 

tion,"  although  he  states  that  in  the  much-mutating  Drosophila  the 
bulk  of  the  mutations  occur  "at  or  very  near  the  maturation  stage." 
In  parthenogenetic  reproduction  in  Cladocera,  with  only  one  matura- 
tion division  (and  that  a  non-reduction  division)  the  probability  of 
mutation  would  seem  further  greatly  reduced.  Hence,  so  far  as  one 
may  judge,  mutations  in  Cladocera  should  be  expected  to  be  much 
less  frequent  than  in  Drosophila.  In  the  latter,  Muller  and  Alten- 
burg  (1919)  state  that  mutation  in  a  particular  factor  is  infrequent. 
From  their  data,1  mutation  in  a  particular  factor  occurs  about  once 
in  50,000  generations.  Yet  this  is  the  much-mutating  Drosophila. 
Hence,  if  one  may  judge  from  the  evidence  from  Drosophila,  mutation 
in  parthenogenetic  reproduction  in  Cladocera  should  be  very  rare. 

The  selection  experiment  with  Line  757  covered  181  generations. 
If  larger  mutations  (affecting  reactiveness  to  light)  occurred,  there 
must  have  been  one  (at  least)  in  the  plus  strain  and  two  (at  least) 
in  181  generations  in  the  minus  strain,  a  surprisingly  high  incidence. 
It  is  possible  that  several  genetic  factors  are  concerned  in  the  photo- 
tropism  of  Simocephalus,  though  McEwan  (1918)  found  that  a  single 
mutant  factor  in  "tan"  Drosophila  profoundly  modified  its  photo- 
tropism.  But  even  if  several  factors  are  concerned  in  the  present 
case,  at  the  rate  of  one  mutation  in  a  particular  factor  in  50,000 
generations  (for  which  Muller  and  Altenburg  give  data  for  Droso- 
phila), the  rate  of  assumed  mutation  in  Line  757  would  seem  most 
excessive.  If  one  disregards  the  possible,  though  improbable,  muta- 
tion in  the  plus  strain  of  Line  740,  and  the  possible,  though  unproven, 
genetic  modifications  in  Lines  689,  691,  711,  719,  762,  794,  and  795, 
of  the  Cladocera  lines  subjected  to  selection,  Line  757  alone  showed 
mutation.  If  larger  mutations  are  to  account  for  the  result  with 
Line  757,  mutation  must  have  been  extremely  frequent  in  this  line 
and  rare  in  the  other  Cladocera  lines  studied. 

It  seems,  then,  that  there  is  difficulty  in  explaining  so  many 
genetic  changes  as  must  have  occurred  in  Line  757  during  a  relatively 
short  period  as  due  to  larger  mutations,  such  as  have  been  worked  out 
for  Drosophila. 

If  one  seeks  in  the  curves  (figure  18s)  for  possible  points  of 
occurrence  of  larger  mutations,  three  points  at  which  such  seem 
more  nearly  possible  than  elsewhere  are  seen  in  the  curves,  about 
April  1913  for  the  minus  strain,  about  January  1915  for  the  plus 
strain,  and  about  August  1915  for  a  second  possible  mutation  in  the 
minus  strain.  But  the  data  by  broods  do  not  indicate  points  at 
which  any  mutation  seems  to  have  occurred.  It  is  possible,  of 
course,  that  the  rather  widely  fluctuating  averages  by  broods  might 
tend  to  mask  the  occurrence  of  a  mutation,  but  even  so  one  would 

1  Conklin  criticizes  Muller  and  Altenburg's  statement  regarding  the  frequency  of  mutation, 
but  even  if  this  statement  is  unwarranted  it  will  still  readily  be  granted  that  mutation  in  a 
particular  factor  is  infrequent. 


A   PHYSIOLOGICAL  CHARACTER.  159 

expect  a  mutation  of  considerable  import  to  appear  evident  very 
quickly  in  the  brood  averages  and  one  would  seem  justified  in  expect- 
ing a  mutation  of  considerable  import  to  produce  a  marked  and 
immediate  change  in  the  two-month  curve  for  the  strain  affected. 
But  such  marked  and  immediate  shifts  are  not  found  in  the  curves 
(figure  18fi),  except  possibly  in  the  Line  757  minus  curve  following 
August  1915,  and  even  there  a  mutation  seems  not  to  have  occurred 
(see  below). 

Since  under  the  influence  of  the  first  two  larger  mutations  pro- 
visionally assumed  to  have  occurred  (but  for  which  no  satisfactory 
locations  can  be  found)  the  curves  do  not  show  great  divergence,  in 
comparison  with  their  later  divergence,  the  wide  divergence  between 
the  curves  for  the  two  strains  during  the  latter  part  of  the  experiment 
seems  to  call  for  a  mutation  of  large  moment  if  the  result  is  to  be 
explained  by  larger  mutations.  The  divergence  in  the  curves  after 
the  third  point  (August  1915),  at  which  a  larger  mutation  may 
possibly  have  occurred,  seems  somewhat  pronounced,  but  does  not 
appear  abrupt  enough  to  have  been  due  to  the  influence  of  a  single 
genetic  change,  unless  the  influence  of  this  mutation  was  masked  for 
a  time  by  a  pronounced  influence  of  environmental  factors.  It  is, 
of  course,  possible  that  this  may  have  been  the  case. 

If,  however,  it  is  necessary  to  concede  a  marked  influence  of 
environmental  factors,  a  more  logical  explanation  of  the  curve  for 
the  minus  strain  would  seem  to  lie  in  the  interpretation  that  the  irreg- 
ularities in  the  curve  for  the  minus  strain  during  the  period  June 
1914-August  1915  were  due  entirely  to  marked  local  environmental 
influences.  There  seems  no  logical  objection  to  this  explanation. 

Evidence  that  this  is  the  correct  interpretation  is  seen  in  the  wide 
fluctuations,  parallel  for  the  most  part  to  those  for  the  minus  strain, 
in  the  Line  757  plus  strain  for  the  same  period.1  Further  evidence 
that  this  may  be  the  correct  interpretation  is  seen  in  the  fact  that 
fluctuations  similar  to  those  found  in  the  strains  of  Line  757  during 
the  period  under  discussion  are  seen  during  certain  periods  in  other 
strains,  e.  g.,  in  strains  of  Lines  695  and  740,  in  which  genetic  changes 
obviously  had  not  occurred  and  in  which  only  local  environmental 
influences  seem  to  account  for  the  facts. 

During  this  time  of  great  fluctuation  in  mean  reaction-times 
in  the  two  strains  of  Line  757,  there  were  three  relatively  very  low 
points  in  the  Line  757  minus  curve.  Except  for  these  low  points, 
the  curve  for  the  minus  strain  is  seen  to  have  held  to  a  fairly 
consistent  course  throughout  the  entire  experiment  (except  for  one 
two-month  period,  December  1916- January  1917,  in  which  local 

1  Except  for  the  June- July  1914  period  the  fluctuations  in  the  curves  for  the  two  strains  of  Line 
757  are  remarkably  similar,  though  it  is  obvious  that  the  Line  757  minus  strain  was  somewhat 
more  influenced  by  environmental  factors  than  the  plus  strain  (figure  18s).  It  is  interesting  to 
note  that  this  June-July  1914  period  is  the  only  period  of  the  entire  experiment  in  which  the 
minus  strain  of  Line  757  appeared  more  reactive  than  the  plus  strain. 


160  SELECTION   IN   CLADOCERA   ON   THE   BASIS   OF 

environmental  influences  obviously  were  the  determining  factor).  If, 
assuming  that  environmental  influences  are  responsible  for  the 
fluctuations  during  parts  of  1914  and  1915,  one  momentarily  dis- 
regards entirely  the  curves  for  the  Line  757  strains  during  this  period 
of  irregularities,  the  curves  for  the  two  strains  following  this  period 
are  seen  to  be  mere  resumptions  of  the  courses  of  the  curves  previous 
to  this  period. 

Hence  it  is  seen  that  if  it  is  necessary  to  assume  that  environ- 
mental factors  exercised  a  determining  influence,  they  may  account 
in  full  for  the  deviations  in  the  curves  from  their  general  courses 
during  parts  of  1914  and  1915,  and  there  seems  no  necessity  for,  nor 
ground  for,  the  assumption  of  a  large  mutation  in  the  minus  strain 
of  Line  757  about  August  1915;  and,  although  this  is  the  only  point 
in  the  curves  at  which  a  mutation  of  considerable  moment  may 
seem  to  have  occurred,  the  writer  believes  that  another  interpreta- 
tion more  logically  accounts  for  the  facts  of  the  case. 

It  therefore  appears  that  the  point  of  occurrence  of  any  really 
large  mutation  can  not  be  satisfactorily  located.  Since  the  divergence 
in  reactiveness  between  the  two  strains  of  Line  757  became  very 
large  and  mutations  of  considerable  moment  can  not  be  located, 
larger  mutations  do  not  seem  to  account  for  the  result  in  Line  757. 
When  one  considers  the  further  fact  that  a  minimum  of  three  larger 
mutations  would  be  required  to  explain  the  case  on  this  basis,  and 
that,  by  analogy  with  the  Drosophila  case,  this  would  call  for  an 
extremely  high  rate  of  mutation,  the  plausibility  of  this  explanation 
still  further  decreases. 

The  data  for  Line  757  readily  permit  of  the  interpretation  of 
the  result  as  having  been  due  to  imperceptibly1  small  changes2 
resulting,  through  selection,  in  cumulative  effects  in  definite  direc- 
tions. The  result  as  shown  in  the  curves  (figures  18B  and  19)  readily 
lends  itself  to  this  explanation,  assuming  that  the  writer's  inter- 
pretation is  correct  and  that  the  irregular  portions  of  the  curves 
(during  parts  of  1914  and  1915)  are  really  due  to  disturbing  en- 

1  At  any  rate,  their  occurrence  was  unperceived.     Reaction-time  in  this  study  is  read  off  in 
seconds.    The  character  studied,  reactiveness  to  light,  would  seem  ideal  for  detecting  minute 
genetic  changes  in  that  the  smallest  changes  in  reactiveness  to  light  should  presumably  be  de- 
tected.    But  the  profound  influence  of  environment  upon  reactiveness  proved  extremely  dis- 
turbing and  only  by  averaging  large  numbers  of  individual  reaction-times  could  modifications 
in  reactivoness  be  detected.     This  was  a  distinct  disappointment.     In  this  study,  because  of 
these  fluctuations  in  reaction-time,  every  case  in  which  there  seems  any  reasonable  doubt  as  to 
an  effect  of  selection  is  refused  credit  in  conclusions  favoring  an  influence  of  selection,  but  in 
no  case  are  the  full  experimental  results  withheld  from  the  reader. 

2  These  changes,  one  believes,  arose  quite  independently  of  selection,  but  were  utilized 
through  selection  in  building  up  the  differences  in  reactiveness  between  the  plus  and  minus 
strains.     Selection  does  not  cause  genetic  change.     It  merely  seizes  upon  modifications  of  the 
character  used  in  selection  as  they  occur,  and  in  the  case  of  plural  genetic  changes  may  build 
up  differences  between  selected  strains.     However,  retention  (through  selection)  of  an  heredi- 
tarily changed  character,  in  so  far  as  it  preserves  the  material  which  may  undergo  further  genetic 
variation,  makes  possible  a  further  change  in  the  same  direction  in  the  same  character.    Selection, 
although  not  a  cause  of  variation,  thus  enables  a  deviation  to  be  built  up  in  the  character  utilized 
in  selection  if  genetic  changes  occur  in  the  direction  sought. 


A   PHYSIOLOGICAL   CHARACTER.  161 

vironmental  influences  as  the  peculiar  results  for  the  December  1916- 
January  1917  period  clearly  are. 

Except  for  the  fluctuations  in  the  curves  for  the  period  men- 
tioned, the  curves  for  the  two  strains  of  Line  757  diverge  rather 
gradually  and  somewhat  uniformly.  There  are  considerable  irregu- 
larities in  the  curve  for  the  plus  strain,  both  during  the  period  when 
irregularities  occurred  in  the  minus  strain  and  elsewhere,  but  despite 
irregularities  the  general  appearance  of  this  curve  strongly  suggests 
that  its  true  course  is  approximately  a  straight  line.  The  general 
course  of  the  curve  for  the  minus  strain  less  strikingly  appears  to  be 
a  straight  line,  but,  aside  from  the  irregularities  already  pointed  out 
and  accounted  for,  this  curve  is  as  regular  as  most  curves  based  upon 
similar  biological  material. 

The  courses  of  the  curves  at  the  end  of  selection  are  such  as 
to  suggest  that  the  divergence  in  reactiveness  between  the  two 
strains  was  still  increasing  when  the  experiment  closed. 

Many  workers  would  prefer  to  call  these  small  genetic  changes 
mutations,  but  they  are  indistinguishable  from  continuous  gradual 
changes  and  must  have  occurred  with  a  frequency  quite  unknown  in 
forms  in  which  the  points  of  occurrence  of  definite  mutations  have 
been  recognized.  Jennings  (1916,  p.  526)  applies  the  term  mutation 
to  such  frequent  small  changes  in  morphological  characters  in 
protozoa,  and  Hegner  (1919)  calls  such  small  structural  changes 
(arising  in  his  Ar cello)  mutations.  But  the  complicated  and  less 
well-known  conditions  of  the  nuclear  material  in  protozoa  and  the 
direct  continuity  between  the  cytoplasm,  which  one  may  think  of  as 
soma,  of  the  parent  and  that  of  offspring  lead  one  to  feel  that  at 
least  there  is  no  positive  assurance  that  conditions  in  protozoa  are 
entirely  comparable  with  metozoa. 

In  parthenogenetic  Cladocera,  without  the  reduction  matura- 
tion division  and  apparently  lacking  in  unequal  distribution  of 
nuclear  elements,  such  as  may  occur  in  division  in  protozoa  (Hegner, 
1919),  frequent  mutation  would  not  seem  to  be  expected,  since  such 
frequent  mutation  as  would  be  called  for  in  the  present  case  has  not 
been  recognized  in  metazoa  and  the  mutations  which  have  been  most 
frequent  have  generally  appeared  to  be  associated  with  maturation. 
So  many  mutations  through  a  lapse  of  only  181  generations  would 
call  for  mutations  at  a  truly  phenomenal  rate.  However,  the  matter 
of  a  name  for  the  genetic  change  merely  turns  upon  one's  definition 
of  a  mutation.  The  facts  of  the  present  case  are  in  nowise  altered, 
whether  the  small  changes  in  character  are  termed  mutations  or 
given  other  designation.  It  is  possible  that  modifications  of  genes 
are  frequently  of  a  low  order  and  consequently  not  generally  recog- 
nized; further,  that  although  they  are  not  to  be  found  wherever 
sought,  once  such  genetic  change  is  initiated,  such  variations  may 
be  frequent  in  occurrence. 


162  SELECTION   IN    CLADOCERA   ON   THE   BASIS   OF 

The  explanation  of  the  results  of  selection  in  this  material  as 
having  been  due  to  many  small  genetic  changes  is  suggested  with 
extreme  caution.  There  is  the  objection  that  such  an  explanation 
runs  counter  to  the  most  acceptable  explanation  for  most  cases  of 
effective  selection  within  the  pure  line  and  that  many  cases  for 
which  this  explanation  has  been  invoked  have  later  yielded  to 
analysis  on  the  basis  of  demonstrable  mutation  or  genetic  segrega- 
tion. But  the  objections  to  explanation  of  the  present  case  as  due 
to  segregation  or  larger  mutations  seem  even  more  weighty  (unless 
one  assumes  a  novel  and  quite  unknown  type  of  segregation  in  this 
material),  and  it  is  believed  that  gradual  modification  of  the  factor  or 
factors  involved  in  the  present  case  must  receive  serious  consideration. 

Since  changes  in  a  character  seem  to  occur  with  such  frequency 
in  some  clones  of  Cladocera,  material  in  which  because  of  uniparental 
inheritance  and  lack  of  reduction  frequent  genetic  changes  would 
seem  not  to  be  expected,  one  wonders  if  this  type  of  organism  is 
unique  among  metazoa  and  if  such  genetic  changes  may  not  likewise 
be  found  to  occur  elsewhere.  In  other  words,  may  not  selection  be 
found  equally  successful  in  some  other  material  and  may  not  the 
case  for  selection  be  more  hopeful  than  is  generally  supposed?  In 
any  material  in  which  improvement  or  change  in  a  character  is 
sought,  genetic  variation  may  (or  may  not)  be  encountered.  If 
genetic  variation  occurs  in  the  character  studied  and  in  the  direction 
sought,  selection  (though  not  a  cause  of  the  variations)  may  be  a 
means  of  utilizing  the  variations  in  accomplishing  the  end  sought. 

The  pure-line  concept,  set  forth  by  Jordan  and  elaborated  by 
Johannsen  and  his  followers,  has  been  extended  until  it  has  been 
shown  to  have  wide  application.  Whether  it  may  become  universal 
in  its  application  remains  to  be  seen.  The  writer  believes  that  such 
cases  as  the  present  are  crucial  in  their  relation  to  the  universal 
validity  of  the  theory,  and  that  if  they  are  not  (or  until  they  are) 
brought  into  line  this  concept  can  not  attain  the  dignity  of  a  law. 
It  is  for  this  reason  that  the  present  case  has  been  presented  in  so 
much  detail. 

Whatever  explanation  one  finds  acceptable,  the  facts  are  that 
the  effects  of  selection  within  Line  757  are  due  both  to  an  increased 
reactiveness  in  the  plus  strain  of  Line  757  and  a  decreased  reactive- 
ness  in  the  minus  strain  of  the  same  line  (both  relative  to  the  corre- 
sponding strains  of  the  other  lines  of  Simocephalus) ,  the  two  modifica- 
tions working  together  to  produce  a  large  difference  in  reactiveness 
between  the  two  strains  of  Line  757;  and  that  this  difference  was 
permanent  or  at  least  persisted  for  112  generations  after  selection 
was  discontinued. 


A   PHYSIOLOGICAL   CHARACTER.  163 


SUMMARY. 

1.  In  experiments  undertaken  to  get  additional  data  on  the 
effects  of  selection  within  the  pure  line,   Cladocera  material  was 
chosen  because  of  certain  advantages:  (a)  it  reproduces  rapidly,  (6) 
is  readily  handled  in  the  laboratory,  (c)  under  controlled  conditions 
it  is  exclusively  parthenogenetic,  and   (d)  in  its  parthenogenetic 
reproduction  there  is  a  single  maturation  division  without  reduction, 
so  that  presumably  it  presents  none  of  the  complications  present  in 
material  in  which  segregation   occurs   during   maturation  and  in 
which  fertilization  occurs. 

2.  Selection  was  based  upon  a  physiological  character:  reactive- 
ness  to  directive  light  stimulation. 

3.  The  material  consisted  of  three  species:  Daphnia  pulex,  D. 
longispina,  and  Simocephalus  exspinosus,  collected  at  different  times 
from  three  ponds  at  Cold  Spring  Harbor. 

4.  The  animals  were  reared  in  200-c.  c.  wide-mouthed  bottles 
with  about  100  c.  c.  of  culture-water  which  was  ordinarily  unchanged 
from  the  time  a  young  female  was  placed  in  the  bottle  until  she 
produced  young. 

5.  Selections  were  made  during  the  first  day  of  life  of  the  young 
daphnids.     The  plus  and  minus  strains  of  each  line  came  from  a 
single  progenitor. 

6.  The  selections  were  conducted  with  light  intensities  of  ap- 
proximately 120  candle-meters. 

7.  Environmental  factors  modified  reaction-time;  but,  presum- 
ably, these  were  horizontal  influences  equally  operative  in  both  strains 
and  with  all  the  individuals  tested  in  making  each  selection,  and 
hence  did  not  influence  the  selections  per  se. 

The  relation  between  environmental  conditions  and  both  re- 
action-time and  vigor  is  considered  in  some  detail. 

(a)  There  are  shown  to  be  low  correlations  between  the  tempera- 
ture of  the  water  in  the  experimental  tank  and  reaction-time  of 
young  daphnids.  These  correlations  are  generally  positive,  meaning 
that  reactiveness  is  decreased  with  increase  in  temperature.  This 
result  was  not  anticipated.  It  is  suggested  that  where  this  relation 
holds  it  may  be  due  to  greater  oxygen-content  in  the  water  of  the 
experimental  tank  with  lower  temperatures. 

(6)  Some  substance  exhaled  from  the  observer's  breath,  pre- 
sumably CO2,  influences  reactiveness.  Frequent  changes  of  the 
water  in  the  experimental  tank  reduced  this  influence  to  a  minimum. 

(c)  Relatively  temporary  chemical  (?)  differences  in  the  water 
used  in  the  experimental  tank  sometimes  influences  reaction-time  to 
a  marked  degree. 


164  SELECTION   IN   CLADOCERA   ON   THE    BASIS   OF 

(d)  Negatively  reacting  individuals  appear  under  circumstances 
suggesting  that  their  occurrence  is  the  result  of  peculiar  local  environ- 
mental conditions  affecting  the  water  of  the  experimental  tank. 

(e)  Broad  up-and-down  movements  in  the  reaction-time  curves 
are  presumably  largely  reflections  of  underlying  (as  distinguished 
from  more  temporary)  changes  in  content  of  the  pond-water,  though 
in  some  cases  temperature  influences  are  contributory.     More  local 
contemporaneous  changes  are  clearly  produced  by  factors  other  than 
temperature,  presumably  changed  constituents  of  the  culture-water 
and  the  water  used  in  the  experimental  tank. 

(/)  Slight  seasonal  (winter  and  summer)  changes  in  reactiveness 
are  attributed  to  direct  temperature  influences  and  to  the  indirect 
influence  of  temperature  as  affecting  the  content  of  the  culture-water 
as  well  as  the  water  used  in  the  experimental  tank. 

(g)  Following  July  1913,  and  extending  throughout  the  re- 
mainder of  the  experiment,  there  was  a  gradual  increase  in  the  re- 
activeness of  all  the  strains  of  Simocephalus.  That  this  is  not  due 
to  the  cumulative  effect  of  favorable  conditions  in  the  laboratory  is 
indicated:  (1)  by  the  fact  that  for  several  months  after  selection  was 
begun  with  the  older  lines  of  Simocephalus  there  was  a  general 
decrease  in  reactiveness ;  (2)  that  the  increase  in  reactiveness  affected 
all  strains  at  the  same  time,  though  they  had  been  in  the  laboratory 
for  different  periods  of  time ;  and  (3)  that  the  reaction-time  curves  for 
the  newer  Simocephalus  lines  (Lines  794,  795,  and  796)  start  at  and 
follow  approximately  the  same  levels  as  were  attained  by  the  older 
lines  for  contemporaneous  two-month  periods. 

(h)  Because  of  the  fact  that  the  two  strains  of  a  selected  line 
ordinarily  reproduce  on  different  days,  culture-water  from  different 
collections  was  usually  used  with  the  two  related  strains.  This 
unavoidably  somewhat  differential  treatment  is  believed  to  account 
for  some  of  the  independent  shifts  in  reaction-time  means  affecting 
the  two  strains  of  the  same  line.  One  such  shift,  for  a  time,  materially 
cut  down  the  difference  in  reaction-time  means  between  the  two 
strains  of  Line  757. 

(i)  Relatively  local  environmental  influences  are  believed  to 
explain  certain  abrupt  shifts  in  the  reaction-time  curves  and  in 
reproductive  indices  which  some  workers  with  Cladocera  might  be 
inclined  to  ascribe  to  "  depression  periods." 

(j)  Environmental  conditions  obviously  influence  the  vigor  of 
Cladocera. 

(ft)  The  "  reproductive  index "  is  believed  to  be  a  safe  measure 
of  general  vigor  and  probably  of  general  muscular  activity  as  well. 

(I)  Similar  effects  of  environmental  conditions  are  seen  in  many 
coincident  fluctuations  in  vigor  of  the  two  strains  of  the  same  line, 
of  all  or  most  of  the  strains  of  the  same  species,  and  even  of  many  of 
the  strains  of  both  Simocephalus  and  Daphnia. 


A   PHYSIOLOGICAL   CHARACTER.  165 

(m)  Independent  fluctuations  in  vigor,  like  the  independent 
fluctuations  in  reaction-time  affecting  related  strains,  are  believed  to 
be  due  to  the  somewhat  differential  treatment  of  the  material. 

(ri)  This  factor  is  also  believed  to  account  for  the  generally 
greater  divergence  between  the  two  strains  of  a  line  during  later 
periods  than  at  the  beginning  of  selection. 

(o)  There  is  not  a  generally  increased  reproductive  index  in 
S.  exspinosus  to  correspond  with  the  increased  reactiveness  of  this 
species. 

(p)  Selection  did  not  produce  plus  strains  of  increasingly  greater 
vigor  as  contrasted  with  minus  strains  of  less  vigor,  either  when 
measured  by  the  numbers  of  plus  and  minus  strains  which  died  out, 
by  the  total  numbers  of  generations  of  descent,  or  by  the  reproduc- 
tive index. 

8.  Sometimes  the  plus  and  minus  strains  of  the  same  line  repro- 
duced on  and  were  tested  on  the  same  day.     The  " same-day-brood" 
data  is  more  trustworthy,  because  of  more  uniform  environmental 
conditions,  than  data  not  simultaneously  obtained,  and  is  given 
separate  treatment. 

9.  Reaction- time  means  were  obtained  of  large  numbers  of 
young  from  mothers  of  each  of  the  two  strains  of  a  line  reared  simul- 
taneously and  tested  on  the  same  day.     These  "test"  series  gave 
averages  obtained  from  several  hundred  individuals  reared  and  sub- 
jected to  stimulation  by  light  under  as  nearly  uniform  conditions  as 
possible,  and  served  to  check  up  the  effect  or  lack  of  effect  of  selection 
within  individual  lines. 

10.  Reaction-time  is  slightly,  if  at  all,  correlated  with  reproduc- 
tive vigor.     This  is  a  point  of  interest,  since  the  minus  strains  in 
general  had  a  somewhat  lower  reproductive  index  than  the  corre- 
sponding plus  strains. 

1 1 .  The  possibility  that  the  method  of  selection  might  have  been 
inadequate  is  discussed  and  evidence  given  which  it  is  believed 
vindicates  the  method  used.     Were  the  method  of  selection  greatly 
at  fault  the  results  obtained  with  some  of  the  lines,  particularly  with 
Line  757,  could  not  have  been  possible. 

12.  In  general,  the  plus  strains  showed  somewhat  greater  repro- 
ductive vigor  (measured  by  reproductive  indices)   than  the  corre- 
sponding minus  strains.     But  this  effect  was  not  cumulative  and 
selection  did  not  result  in  plus  strains  of  increasingly  greater  vigor 
than  corresponding  minus  strains. 

13.  There  are  differences  between  the  behavior  of  the  different 
species   in   the   experimental   tank.     S.    exspinosus   was   generally 
relatively  slightly  reactive  compared  with  the  two  species  of  Daphnia. 
Many  individuals  of  this  species  (and  a  few  of  the  species  of  Daphnia) 
failed  to  reach  either  end  of  the  experimental  tank  during  the  15- 
minute  period  of  the  selection  test. 


166  SELECTION   IN   CLADOCERA   ON   THE    BASIS   OF 

14.  The  data  for  the  over-time  individuals  presented  a  serious 
difficulty.     In  many  cases,  especially  with  the  Simocephalus  lines, 
they  constituted  a  considerable  portion,  and  a  most  significant  por- 
tion, of  the  reaction-time  data.     Over-time  individuals  were  arbi- 
trarily assigned  a  reaction-time  of  900  seconds  and  treated  as  though 
they  had  reached  an  end  of  the  tank  within  the  period  of  the  test. 
This  expedient  really   minimized   the  rightful  influence   of  these 
relatively  slightly  reactive  but  highly  significant  individuals. 

15.  Negatively  reacting  individuals  were  very  irregular  in  occur- 
rence and  of  doubtful  significance.     They  were  utilized  in  selection  in 
the  minus  strains  under  the  assumption  that  a  genetic  change  might 
occur  rendering  an  individual  fundamentally  negative  in  its  reactions 
to  light. 

16.  Many  of  the  data  are  published  for  the  benefit  of  workers 
who  might  wish  to  go  over  the  analysis  or  make  further  analysis. 

In  the  treatment  of  the  data  for  the  different  lines  the  following 
results  are  recorded: 

17.  For  Line  695  there  was  not  an  effect  of  selection.    A  marked 
general  influence  of  environmental  factors  upon  reaction-times  is 
observed,  as  is  true  with  most  of  the  other  lines. 

18.  In  Line  689  selection  was  not  effective.     On  the  contrary, 
there  was  a  persistent,  though  slight,  difference  tending  to  indicate 
that  the  minus  strain  was  the  more  reactive. 

19.  In  Line  691  there  was  no  certain  effect  of  selection,  though 
part  of  the  data  suggests  such  an  effect. 

20.  In  Line  711  a  selective  effect  is  strongly  suggested,  though 
not  fully  supported  by  the  same-day-brood  data.     The  evidence  for  a 
genetic  difference  between  the  two  strains  is,  however,  as  good  or 
better  than  that  suggesting  a  genetic  difference  between  the  two 
strains  of  Line  689. 

21.  The  data  in  part  suggest  an  effect  of  selection  in  Line  713, 
but  it  is  improbable  that  such  actually  occurred. 

22.  Some  of  the  early  data  for  Line  714  suggest  an  effect  of 
selection,  but  the  later  data  show  clearly  that  such  had  not  occurred. 

23.  In  Line  719,  as  in  Line  689,  there  is  a  generally  higher 
reaction-time  for  the  plus  strain.     The  different  periods  of  the  data, 
in  spite  of  considerable  fluctuations,  rather  consistently  bear  out  this 
interpretation,  and  genetic  differences  in  opposition  to  selection  are 
believed  to  have  arisen.     Selection  possibly  operated  to  reduce  this 
difference,  which  was  less  marked  during  the  latter  part  of   the 
experiment. 

24.  There  was  no  effect  of  selection  in  Line  751. 

25.  Some  of  the  final  data  for  Line  762  suggest  an  effect  of 
selection,  but  the  data  are  not  sufficient  to  be  at  all  convincing. 


A   PHYSIOLOGICAL   CHARACTER.  167 

26.  In  the  short  experiment  with  Line  766  (derived  from  Line 
762)  there  was  no  effect  of  selection. 

27.  There  was  no  effect  of  selection  in  Line  768.    * 

28.  Most  of  the  data  for  Line  794  suggest  an  effect  of  selection, 
though  the  test  series  did  not  support  this  conclusion  and  effect  of 
selection  must  remain  in  doubt. 

29.  Effect  of  selection  is  strongly  indicated  for  the  final  data 
for  Line  795,  but  in  view  of  the  earlier  differences  in  the  opposite 
direction  and  the  small  amount  of  the  data  which  indicate  an  effect 
of  selection,  the  result  is  questionable. 

30.  There  is  clearly  no  effect  of  selection  in  Line  796. 

31.  Certain  portions  of  the  data  for  Line  740  seem  to  indicate 
consistent  differences  in  reaction-time  for  the  two  strains.     A  possible 
mutation  early  in  the  course  of  the  experiment  was  examined,  but 
ruled   out  as  improbable.     For   14  months  preceding  the  final  3 
months  of  the  experiment,  the   plus   strain  was  somewhat  more 
reactive.  However,  the  data  for  the  final  3  months  of  the  experiment, 
together  with  a  large  test  series,  indicate  a  lack  of  selective  effect. 

32.  A  large  effect  of  selection  is  clearly  indicated  for  Line  757: 
(a)  This   conclusion  is  consistently  attested  by  every  check 

which  was  applied  to  the  data. 

(6)  The  effect  was  due  both  to  an  increased  reactiveness  of  the 
plus  strain  and  a  reduced  reactiveness  in  the  minus  strain 
(relatively  to  the  other  plus  and  minus  strains  of  Simo- 
cephalus) . 

(c)  The  divergence  between  mean  reaction- times  became  large. 

(d)  The  divergence  in  reactiveness  was  permanent,  or,  at  any 
rate,  still  existed  32  months  (112  generations)  after  selection 
was  discontinued. 

(e)  Except  for  the  difference  in  reactiveness  to  light,  and  a 

slightly  lower  reproductive  index  (which  was  not  in  evidence 
during  the  latter  part  of  the  experiment)  in  the  minus 
strain,  there  is  nothing  to  distinguish  the  two  strains. 

33.  It  is  not  believed  that  general  physiological  differences  or 
cytoplasmic  transmission  can  be  appealed  to  as  explaining  the  result, 
particularly  in  view  of  the  apparently  permanent  nature  of  the 
changes  in  reactiveness. 

34.  The  effect  is  presumably  not  due  to  genetic  segregation 
(unless  there  is  assumed  a  novel  method  of  segregation,  for  which 
there  is  no  evidence)  inasmuch  as  at  least  three  such  segregations 
seem  necessary  to  explain  the  result  on  this  basis  and  there  is  no 
chromatic  reduction  in  the  maturation  of  parthenogenetic  eggs  of 
Cladocera. 

35.  A  possible  novel  method  of  segregation  is  suggested.     Such 
an  assumption  would  explain  the  results  with  Line  757  in  accord  with 


168  SELECTION    IN   CLADOCERA. 

the  pure-line  hypothesis,  but  there  is  no  evidence  for  the  occurrence 
in  Cladocera  of  such  unusual  chromatic  behavior. 

36.  The  divergence  in  reaction-time  may  be  explained  as  due 
to  larger  mutations.     But  the  data  do  not  show  points,  with  one 
possible  exception,  at  which  such  mutations  may  presumably  have 
occurred,  and,  assuming  a  minimum  of  3  mutations,  each  mutation 
must  have  produced  its  maximum  effect  long  after  its  occurrence,  a 
fact  not  consistent  with  the  current  conception  of  a  mutation  in  such 
material.     Further,    the   mutations   must   have   occurred   in   both 
strains,  and  since  three  and  probably  more  mutations  are  required, 
this  would  call  for  a  high*  rate  of  mutation  in  this  line. 

37.  The  result  may  be  explained  as  due  to  a  gradual  modifica- 
tion, many  small  changes  (mutations?),  in  the  genetic  factor  or  factors 
involved. 


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1913.  Selection  within  pure  lines  in  Daphnia.    Science,  n.  s.,  37,  p.  272. 

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1908.  Vererbung  bei  ungeschlechtlicher  Fortpflanzung  von  Hydra  grisea.    Jena. 

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1918.  The  reactions  to  light  and  to  gravity  in  Drosophila  and  its  mutants.    Jour. 

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169 


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UNIVERSITY  OF  CALIFORNIA,  SANTA  CRUZ 

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This  book  is  due  on  the  last  DATE  stamped  below. 


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